Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2019	Nov. 23, 2019
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Quarterly Report	true
Document Transition Report	false
Entity Interactive Data Current	Yes
Document Period End Date	Sep. 30, 2019
Document Fiscal Year Focus	2019
Current Fiscal Year End Date	--12-31
Document Fiscal Period Focus	Q3
Entity Registrant Name	Phathom Pharmaceuticals, Inc.
Entity Central Index Key	0001783183
Entity Tax Identification Number	82-4151574
Entity Current Reporting Status	No
Entity Filer Category	Non-accelerated Filer
Entity File Number	001-39094
Entity Shell Company	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Small Business	true
Entity Incorporation, State or Country Code	DE
Entity Address, Address Line One	2150 E. Lake Cook Road
Entity Address, Address Line Two	Suite 800
Entity Address, City or Town	Buffalo Grove
Entity Address, State or Province	IL
Entity Address, Postal Zip Code	60089
City Area Code	(650)
Local Phone Number	325-5156
Entity Common Stock, Shares Outstanding		24,526,537
Title of 12(b) Security	Common Stock, $0.0001 par value per share
Trading Symbol	PHAT
Security Exchange Name	NASDAQ