Cover - USD ($)	12 Months Ended
	Dec. 31, 2020	Mar. 15, 2021	Jun. 30, 2020
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2020
Current Fiscal Year End Date	--12-31
Document Transition Report	false
Entity File Number	001-38295
Entity Registrant Name	X4 PHARMACEUTICALS, INC.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	27-3181608
Entity Address, Address Line One	61 North Beacon Street
Entity Address, Address Line Two	4th Floor
Entity Address, City or Town	Boston
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02134
City Area Code	857
Local Phone Number	529-8300
Title of 12(b) Security	Common Stock
Trading Symbol	XFOR
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
ICFR Auditor Attestation Flag	false
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Shell Company	false
Entity Public Float			$ 144,752,443
Entity Common Stock, Shares Outstanding		17,384,478
Documents Incorporated by Reference	Portions of the registrant’s definitive proxy statement, or the 2021 Proxy Statement, for its 2021 Annual Meeting of Stockholders, which the registrant intends to file pursuant to Regulation 14A with the Securities and Exchange Commission not later than 120 days after the registrant's fiscal year ended December 31, 2020, are incorporated by reference into Part III of this Annual Report on Form 10-K.
Entity Central Index Key	0001501697
Amendment Flag	false
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY