Filed pursuant to Rule 424(b)(3)
SEC File No. 333-258223

PROSPECTUS SUPPLEMENT NO. 1

(to Prospectus dated August 10, 2021)

ironSource Ltd.

133,254,045 CLASS A ORDINARY SHARES

This prospectus supplement updates, amends and supplements the prospectus dated August 10, 2021 (as supplemented or amended from time to time, the “Prospectus”), which forms a part of our Registration Statement on Form F-1 (Registration No. 333-258223). Capitalized terms used in this prospectus supplement and not otherwise defined herein have the meanings specified in the Prospectus.

This prospectus supplement is being filed to update, amend and supplement the information included in the Prospectus with information on ironSource’s second quarter 2021 financial results, which is set forth below.

This prospectus supplement is not complete without the Prospectus. This prospectus supplement should be read in conjunction with the Prospectus, which is to be delivered with this prospectus supplement, and is qualified by reference thereto, except to the extent that the information in this prospectus supplement updates or supersedes the information contained in the Prospectus. Please keep this prospectus supplement with your Prospectus for future reference.

Our Class A ordinary shares are listed on the New York Stock Exchange under the symbol “IS.” On August 23, 2021, the closing price for our Class A ordinary shares on the New York Stock Exchange was $9.01 per share.

Investing in our securities involves a high degree of risk. See “Risk Factors” beginning on page 30 of this Prospectus and other risk factors contained in the documents incorporated by reference therein for a discussion of information that should be considered in connection with an investment in our securities.

Neither the Securities and Exchange Commission, the Israeli Securities Authority nor any state securities commission has approved or disapproved of these securities or determined if the Prospectus or this prospectus supplement is truthful or complete. Any representation to the contrary is a criminal offense.

The date of this prospectus supplement is August 24, 2021.

IRONSOURCE LTD.

CONDENSED CONSOLIDATED BALANCE SHEETS

(U.S. dollars in thousands, except for number of shares and par value)

(Unaudited)

	June 30,		December 31,
	2021		2020
Assets
Current assets:
Cash and cash equivalents	$	706,797	$	200,672
Short-term deposits		—		17,627
Accounts receivable, net of allowances of $793 and $724 as of June 30, 2021 and December 31, 2020, respectively		186,844		151,503
Other current assets		35,654		15,711
Total current assets		929,295		385,513
Long-term restricted cash		2,881		2,415
Deferred tax assets		3,412		161
Operating lease right-of-use asset		33,571		36,780
Property, equipment and software, net		24,646		23,077
Investment in equity securities		20,000		—
Goodwill		205,842		79,156
Intangible assets, net		31,344		8,084
Other non-current assets		4,782		650
Total assets	$	1,255,773	$	535,836

F-1

IRONSOURCE LTD.

CONDENSED CONSOLIDATED BALANCE SHEETS (continued)

(U.S. dollars in thousands, except for number of shares and par value)

(Unaudited)

	June 30,			December 31,
	2021			2020
Liabilities and shareholders’ equity
Current liabilities:
Accounts payable	$	178,884		$	155,476
Current maturities of long-term loan		—			9,725
Operating lease liabilities		7,334			7,429
Other current liabilities		31,771			34,034
Total current liabilities		217,989			206,664
Long-term loan, net of current maturities		—			74,684
Deferred tax liabilities		1,037			2,521
Long-term operating lease liabilities		29,969			32,241
Other non-current liabilities		1,761			280
Total liabilities		250,756			316,390
Commitments and contingencies
Shareholders’ equity:
Class A ordinary share, no par value; 10,000,000,000 shares authorized at June 30, 2021 and December 31, 2020; 627,508,621 and 320,133,022 issued and outstanding at June 30, 2021 and December 31, 2020, respectively(1)		—			—
Class B ordinary share, no par value; 1,500,000,000 shares authorized at June 30, 2021 and December 31, 2020; 385,108,621 and 320,133,022 issued and outstanding at June 30, 2021 and December 31, 2020, respectively(1)		—			—
2019 ordinary shares, NIS 0.01 par value, 25,006,298 authorized, issued and outstanding at December 31, 2020		—			72
Treasury shares, at cost, 6,745,955 Class A ordinary shares held at June 30, 2021		(67,460	)		—
Additional paid-in capital(1)		985,106			152,251
Retained earnings		87,371			67,123
Total shareholders’ equity		1,005,017			219,446
Total liabilities and shareholders’ equity	$	1,255,773		$	535,836

(1) Per share amounts have been adjusted, on a retroactive basis, for all periods presented, to reflect both the distribution of Class B ordinary shares and the Stock Split, together representing a ratio of 9.98 of each share.

F-2

IRONSOURCE LTD.

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS

(U.S. dollars in thousands, except share and per share amounts)

(Unaudited)

	Three Months Ended June 30,				Six Months Ended June 30,
	2021		2020		2021		2020
Revenue	$	135,036	$	73,958	$	254,749	$	135,164
Cost of revenue		22,765		13,556		42,905		25,103
Gross profit		112,271		60,402		211,844		110,061
Operating expenses:
Research and development		23,161		11,471		43,571		21,600
Sales and marketing		52,181		28,239		100,902		47,411
General and administrative		20,686		6,781		36,233		13,483
Total operating expenses		96,028		46,491		180,706		82,494
Income from operations		16,243		13,911		31,138		27,567
Financial expenses, net		977		1,204		2,006		2,176
Income from continuing operations before income taxes		15,266		12,707		29,132		25,391
Income taxes		5,262		1,994		8,884		3,788
Income from continuing operations, net of income taxes		10,004		10,713		20,248		21,603
Income from discontinued operations, net of income taxes		—		12,487		—		24,788
Net income	$	10,004	$	23,200	$	20,248	$	46,391
Basic net income per ordinary share:(1)
Continuing operations		0.01		0.01		0.02		0.02
Discontinued operations		—		0.01		—		0.03
Basic net income per ordinary share	$	0.01	$	0.02	$	0.02	$	0.05
Weighted-average ordinary shares outstanding—basic		658,950,556		636,188,253		652,122,890		635,000,653
Diluted net income per ordinary share:(1)
Continuing operations		0.01		0.01		0.02		0.02
Discontinued operations		—		0.01		—		0.03
Diluted net income per ordinary share	$	0.01	$	0.02	$	0.02	$	0.05
Weighted-average ordinary shares outstanding—diluted		746,974,212		681,273,631		729,329,729		674,803,455

(1) Per share amounts have been adjusted, on a retroactive basis, for all periods presented, to reflect both the distribution of Class B ordinary shares and the Stock Split, together representing a ratio of 9.98 of each share.

F-3

IRONSOURCE LTD.

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS

(U.S. dollars in thousands)

(Unaudited)

	Three months Ended June 30,						Six months Ended June 30,
	2021			2020			2021			2020
	(Unaudited)						(Unaudited)
Cash flows from operating Activities
Net income from continuing operations	$	10,004		$	10,713		$	20,248		$	21,603
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization		5,874			4,138			11,217			8,139
Share-based compensation expenses		20,664			2,509			37,474			5,341
Non-cash lease expense		615			(369	)		842			(10	)
Effect of exchange rate changes on cash and cash equivalents and restricted cash		1,107			(723	)		(139	)		448
Interest accrued and other financial expenses		521			(22	)		628			108
Deferred income taxes, net		99			(413	)		(728	)		(695	)
Changes in operating assets and liabilities:
Accounts receivable		(38,015	)		(8,411	)		(38,866	)		(5,263	)
Other current assets		(3,894	)		3,700			(18,644	)		1,780
Other non-current assets		(4,740	)		(1,563	)		(8,037	)		(1,542	)
Accounts payable		27,025			4			20,368			2,935
Other current liabilities		(7,404	)		5,604			(1,553	)		(416	)
Other long-term liabilities		438			34			1,481			42
Net cash provided by continuing operating activities		12,294			15,201			24,291			32,470
Net cash provided by (used in) discontinued operating activities		—			14,487			(5,168	)		30,974
Net cash provided by operating activities		12,294			29,688			19,123			63,444
Cash flows from investing activities
Purchase of property, plant and equipment		(287	)		(291	)		(760	)		(831	)
Capitalized software development costs		(2,587	)		(2,836	)		(5,602	)		(5,765	)
Purchase of intangible assets		—			—			(1,950	)		—
Acquisitions, net of cash acquired		—			—			(90,184	)		—
Purchase of equity investment		(20,000	)		—			(20,000	)		—
Investments in short-term deposits		—			—			—			(5,000	)
Maturities of short-term deposits		—			—			17,590			8,100
Net cash used in continuing investing activities		(22,874	)		(3,127	)		(100,906	)		(3,496	)
Net cash used in discontinued investing activities		—			(1,488	)		—			(2,940	)
Net cash used in investing activities		(22,874	)		(4,615	)		(100,906	)		(6,436	)

F-4

IRONSOURCE LTD.

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (continued)

(U.S. dollars in thousands)

(Unaudited)

	Three months Ended June 30,						Six months Ended June 30,
	2021			2020			2021			2020
	(Unaudited)						(Unaudited)
Cash flows from financing activities
Repayment of long-term loan		(82,500	)		(1,250	)		(85,000	)		(2,500	)
Proceeds from Recapitalization transaction, net		673,953						672,893			—
Exercise of options		45			272			342			661
Net cash provided (used in) continuing financing activities		591,498			(978	)		588,235			(1,839	)
Net cash provided (used in) discontinued financing activities		—			—			—			—
Net cash provided (used in) financing activities		591,498			(978	)		588,235			(1,839	)
Effect of exchange rate changes on cash and cash equivalents and restricted cash		(1,107	)		723			139			(448	)
Net change in cash and cash equivalents and restricted cash		580,918			24,095			506,452			55,169
Cash and cash equivalents and restricted cash at beginning of the period		129,867			121,122			203,087			91,219
Cash and cash equivalents and restricted cash at end of the period	$	709,678		$	145,940		$	709,678		$	145,940

F-5