<?xml version="1.0" encoding="us-ascii"?><InstanceReport xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"><Version>2.4.0.8</Version><ReportLongName>112 - Disclosure - Series B Preferred Stock</ReportLongName><DisplayLabelColumn>true</DisplayLabelColumn><ShowElementNames>false</ShowElementNames><RoundingOption /><HasEmbeddedReports>false</HasEmbeddedReports><Columns><Column FlagID="0"><Id>1</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><CurrencyCode /><FootnoteIndexer /><hasSegments>false</hasSegments><hasScenarios>false</hasScenarios><MCU><KeyName /><CurrencySymbol /><contextRef><ContextID>eol_PE11654---1310-Q0007_STD_181_20130630_0</ContextID><EntitySchema>http://www.sec.gov/CIK</EntitySchema><EntityValue>0001024732</EntityValue><PeriodDisplayName /><PeriodType>duration</PeriodType><PeriodStartDate>2013-01-01T00:00:00</PeriodStartDate><PeriodEndDate>2013-06-30T00:00:00</PeriodEndDate><Segments /><Scenarios /></contextRef><UPS /><CurrencyCode /><OriginalCurrencyCode /></MCU><CurrencySymbol /><Labels><Label Key="CalendarSupplement" Id="0" Label="6 Months Ended" /><Label Key="Calendar" Id="1" Label="Jun. 30, 2013" /></Labels></Column></Columns><Rows><Row FlagID="0"><Id>1</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><Level>4</Level><ElementName>us-gaap_PreferredStockTextBlock</ElementName><ElementPrefix>us-gaap_</ElementPrefix><IsBaseElement>true</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsCalendarTitle>false</IsCalendarTitle><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><PreferredLabelRole>terseLabel</PreferredLabelRole><FootnoteIndexer /><Cells><Cell FlagID="0" ContextID="eol_PE11654---1310-Q0007_STD_181_20130630_0" UnitID=""><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText>&lt;div&gt;
&lt;table style="BORDER-COLLAPSE:COLLAPSE" border="0" cellpadding="0" cellspacing="0" width="100%"&gt;
&lt;tr&gt;
&lt;td width="4%" valign="top" align="left"&gt;&lt;font style="font-family:Times New Roman" size="2"&gt;&lt;b&gt;4.&lt;/b&gt;&lt;/font&gt;&lt;/td&gt;
&lt;td align="left" valign="top"&gt;&lt;font style="font-family:Times New Roman" size="2"&gt;&lt;b&gt;Series B Preferred
Stock&lt;/b&gt;&lt;/font&gt;&lt;/td&gt;
&lt;/tr&gt;
&lt;/table&gt;
&lt;p style="margin-top:6px;margin-bottom:0px; text-indent:4%"&gt;
&lt;font style="font-family:Times New Roman" size="2"&gt;On
January&amp;#xA0;10, 2011, the Company entered into a securities
purchase agreement, with Viscaria Limited, to raise equity
financing to pay a portion of the cash consideration for the
acquisition of iGATE Computer. Under the securities purchase
agreement, the Company agreed to sell, in a private placement, up
to 480,000 shares of newly designated 8.00% Series B Convertible
Participating Preferred Stock, no par value per share (the
&amp;#x201C;Series B Preferred Stock&amp;#x201D;), for an aggregate purchase
price of up to $480 million. On February&amp;#xA0;1, 2011 and
May&amp;#xA0;9, 2011, the Company issued 210,000 shares and 120,000
shares, respectively, of the Series B Preferred Stock for a
consideration of $330 million.&lt;/font&gt;&lt;/p&gt;
&lt;p style="margin-top:12px;margin-bottom:0px; text-indent:4%"&gt;
&lt;font style="font-family:Times New Roman" size="2"&gt;Significant
economic terms of the Series B Preferred Stock include:&lt;/font&gt;&lt;/p&gt;
&lt;p style="font-size:6px;margin-top:0px;margin-bottom:0px"&gt;
&amp;#xA0;&lt;/p&gt;
&lt;table style="BORDER-COLLAPSE:COLLAPSE" border="0" cellpadding="0" cellspacing="0" width="100%"&gt;
&lt;tr&gt;
&lt;td width="5%"&gt;&lt;font size="1"&gt;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td width="2%" valign="top" align="left"&gt;&lt;font style="font-family:Times New Roman" size="2"&gt;&amp;#x2022;&lt;/font&gt;&lt;/td&gt;
&lt;td width="1%" valign="top"&gt;&lt;font size="1"&gt;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td align="left" valign="top"&gt;
&lt;p align="left"&gt;&lt;font style="font-family:Times New Roman" size="2"&gt;accrues cumulative dividends at a rate of 8.00%&amp;#xA0;per annum,
which dividends will be added to the liquidation preference of the
Series B Preferred Stock and compounded quarterly;&lt;/font&gt;&lt;/p&gt;
&lt;/td&gt;
&lt;/tr&gt;
&lt;/table&gt;
&lt;p style="font-size:6px;margin-top:0px;margin-bottom:0px"&gt;
&amp;#xA0;&lt;/p&gt;
&lt;table style="BORDER-COLLAPSE:COLLAPSE" border="0" cellpadding="0" cellspacing="0" width="100%"&gt;
&lt;tr&gt;
&lt;td width="5%"&gt;&lt;font size="1"&gt;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td width="2%" valign="top" align="left"&gt;&lt;font style="font-family:Times New Roman" size="2"&gt;&amp;#x2022;&lt;/font&gt;&lt;/td&gt;
&lt;td width="1%" valign="top"&gt;&lt;font size="1"&gt;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td align="left" valign="top"&gt;
&lt;p align="left"&gt;&lt;font style="font-family:Times New Roman" size="2"&gt;is entitled to participate in dividends and other distributions
payable on the Company&amp;#x2019;s common stock on an as-converted
basis;&lt;/font&gt;&lt;/p&gt;
&lt;/td&gt;
&lt;/tr&gt;
&lt;/table&gt;
&lt;p style="font-size:1px;margin-top:24px;margin-bottom:0px"&gt;
&amp;#xA0;&lt;/p&gt;
&lt;table style="BORDER-COLLAPSE:COLLAPSE" border="0" cellpadding="0" cellspacing="0" width="100%"&gt;
&lt;tr&gt;
&lt;td width="5%"&gt;&lt;font size="1"&gt;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td width="2%" valign="top" align="left"&gt;&lt;font style="font-family:Times New Roman" size="2"&gt;&amp;#x2022;&lt;/font&gt;&lt;/td&gt;
&lt;td width="1%" valign="top"&gt;&lt;font size="1"&gt;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td align="left" valign="top"&gt;
&lt;p align="left"&gt;&lt;font style="font-family:Times New Roman" size="2"&gt;provides for a holder option to convert the outstanding
principal plus accrued and unpaid dividends into the
Company&amp;#x2019;s common stock at any time and from time to time at
an initial conversion price of $20.30 per share (which conversion
price is subject to adjustment in certain circumstances such as the
Company&amp;#x2019;s sale or issuance of shares of common stock is for a
price per share less than the current market price of its common
stock on the date of sale or issue (other than issuances under the
stock option or stock ownership plans), subdivision or combination
of the Company&amp;#x2019;s common stock (e.g. by stock split or stock
dividend), wherein the conversion price in effect will be
proportionately reduced or increased on or after such effective or
record date and merger, reorganization, consolidation or sale of
substantially all of the assets);&lt;/font&gt;&lt;/p&gt;
&lt;/td&gt;
&lt;/tr&gt;
&lt;/table&gt;
&lt;p style="font-size:6px;margin-top:0px;margin-bottom:0px"&gt;
&amp;#xA0;&lt;/p&gt;
&lt;table style="BORDER-COLLAPSE:COLLAPSE" border="0" cellpadding="0" cellspacing="0" width="100%"&gt;
&lt;tr&gt;
&lt;td width="5%"&gt;&lt;font size="1"&gt;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td width="2%" valign="top" align="left"&gt;&lt;font style="font-family:Times New Roman" size="2"&gt;&amp;#x2022;&lt;/font&gt;&lt;/td&gt;
&lt;td width="1%" valign="top"&gt;&lt;font size="1"&gt;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td align="left" valign="top"&gt;
&lt;p align="left"&gt;&lt;font style="font-family:Times New Roman" size="2"&gt;is subject to a Company option to convert the Series B
Preferred Stock into common stock of the Company after 18 months
from the applicable closing date if, among other things, the volume
weighted average price of the Company&amp;#x2019;s common stock exceeds
205% of the then applicable conversion price for a specified period
of time;&lt;/font&gt;&lt;/p&gt;
&lt;/td&gt;
&lt;/tr&gt;
&lt;/table&gt;
&lt;p style="font-size:6px;margin-top:0px;margin-bottom:0px"&gt;
&amp;#xA0;&lt;/p&gt;
&lt;table style="BORDER-COLLAPSE:COLLAPSE" border="0" cellpadding="0" cellspacing="0" width="100%"&gt;
&lt;tr&gt;
&lt;td width="5%"&gt;&lt;font size="1"&gt;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td width="2%" valign="top" align="left"&gt;&lt;font style="font-family:Times New Roman" size="2"&gt;&amp;#x2022;&lt;/font&gt;&lt;/td&gt;
&lt;td width="1%" valign="top"&gt;&lt;font size="1"&gt;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td align="left" valign="top"&gt;
&lt;p align="left"&gt;&lt;font style="font-family:Times New Roman" size="2"&gt;is redeemable for cash at an amount equal to the outstanding
principal plus accrued and unpaid dividends upon the exercise of
the holder&amp;#x2019;s put right at six years from the last occurring
closing date;&lt;/font&gt;&lt;/p&gt;
&lt;/td&gt;
&lt;/tr&gt;
&lt;/table&gt;
&lt;p style="font-size:6px;margin-top:0px;margin-bottom:0px"&gt;
&amp;#xA0;&lt;/p&gt;
&lt;table style="BORDER-COLLAPSE:COLLAPSE" border="0" cellpadding="0" cellspacing="0" width="100%"&gt;
&lt;tr&gt;
&lt;td width="5%"&gt;&lt;font size="1"&gt;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td width="2%" valign="top" align="left"&gt;&lt;font style="font-family:Times New Roman" size="2"&gt;&amp;#x2022;&lt;/font&gt;&lt;/td&gt;
&lt;td width="1%" valign="top"&gt;&lt;font size="1"&gt;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td align="left" valign="top"&gt;
&lt;p align="left"&gt;&lt;font style="font-family:Times New Roman" size="2"&gt;provides that, if the Series B Preferred Stock is not sooner
converted, such preferred stock is subject to a mandatory
conversion into shares of the Company&amp;#x2019;s common stock on the
date that is six years from the applicable closing date (subject to
extension in limited circumstances) unless the holder exercises the
put right described in the immediately preceding bullet point;
and&lt;/font&gt;&lt;/p&gt;
&lt;/td&gt;
&lt;/tr&gt;
&lt;/table&gt;
&lt;p style="font-size:6px;margin-top:0px;margin-bottom:0px"&gt;
&amp;#xA0;&lt;/p&gt;
&lt;table style="BORDER-COLLAPSE:COLLAPSE" border="0" cellpadding="0" cellspacing="0" width="100%"&gt;
&lt;tr&gt;
&lt;td width="5%"&gt;&lt;font size="1"&gt;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td width="2%" valign="top" align="left"&gt;&lt;font style="font-family:Times New Roman" size="2"&gt;&amp;#x2022;&lt;/font&gt;&lt;/td&gt;
&lt;td width="1%" valign="top"&gt;&lt;font size="1"&gt;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td align="left" valign="top"&gt;
&lt;p align="left"&gt;&lt;font style="font-family:Times New Roman" size="2"&gt;provides the holder the right to receive, prior to any payment
in respect of any junior equity securities, the greater of the
outstanding principal plus accrued and unpaid dividends and the
as-converted value upon liquidation of the Company or upon certain
changes of control.&lt;/font&gt;&lt;/p&gt;
&lt;/td&gt;
&lt;/tr&gt;
&lt;/table&gt;
&lt;p style="margin-top:12px;margin-bottom:0px; text-indent:4%"&gt;
&lt;font style="font-family:Times New Roman" size="2"&gt;The Company
incurred issuance costs amounting to $3.4 million which have been
netted against the proceeds received from the issuance of Series B
Preferred Stock. The Series B Preferred Stock is being accreted
over a period of six years. The amount accreted totaled $0.1
million each during the three months ended June&amp;#xA0;30, 2013 and
2012 and $0.2 million each during the six months ended
June&amp;#xA0;30, 2013 and 2012.&lt;/font&gt;&lt;/p&gt;
&lt;p style="margin-top:12px;margin-bottom:0px; text-indent:4%"&gt;
&lt;font style="font-family:Times New Roman" size="2"&gt;The Company is
accruing for cumulative dividends at a rate of 8.00%&amp;#xA0;per
annum, compounded quarterly. The amount of dividends accrued was
$7.8 million and $7.2 million during the three months ended
June&amp;#xA0;30, 2013 and 2012, respectively, and $15.3 million and
$14.2 million during the six months ended June&amp;#xA0;30, 2013 and
2012, respectively.&lt;/font&gt;&lt;/p&gt;
&lt;p style="margin-top:12px;margin-bottom:0px; text-indent:4%"&gt;
&lt;font style="font-family:Times New Roman" size="2"&gt;As of
June&amp;#xA0;30, 2013 and 2012, the shares of Series B Preferred Stock
are potentially convertible into 19.5&amp;#xA0;million and
18.0&amp;#xA0;million shares of common stock, respectively.&lt;/font&gt;&lt;/p&gt;
&lt;/div&gt;</NonNumbericText><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell></Cells><ElementDataType>nonnum:textBlockItemType</ElementDataType><SimpleDataType>na</SimpleDataType><ElementDefenition>The entire disclosure for terms, amounts, nature of changes, rights and privileges, dividends, and other matters related to preferred stock.</ElementDefenition><ElementReferences>No definition available.</ElementReferences><IsTotalLabel>false</IsTotalLabel><UnitID>0</UnitID><Label>Series B Preferred Stock</Label></Row></Rows><Footnotes /><IsEquityReport>false</IsEquityReport><ReportName>Series B Preferred Stock</ReportName><MonetaryRoundingLevel>UnKnown</MonetaryRoundingLevel><SharesRoundingLevel>UnKnown</SharesRoundingLevel><PerShareRoundingLevel>UnKnown</PerShareRoundingLevel><ExchangeRateRoundingLevel>UnKnown</ExchangeRateRoundingLevel><HasCustomUnits>true</HasCustomUnits><IsEmbedReport>false</IsEmbedReport><IsMultiCurrency>false</IsMultiCurrency><ReportType>Sheet</ReportType><RoleURI>http://www.igate.com/taxonomy/role/NotesToFinancialStatementsPreferredStockTextBlock</RoleURI><NumberOfCols>1</NumberOfCols><NumberOfRows>1</NumberOfRows></InstanceReport>
