Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2020	Jul. 31, 2020
Document And Entity Information
Document Type	10-Q
Document Quarterly Report	true
Document Transition Report	false
Document Period End Date	Jun. 30, 2020
Entity File Number	001-35589
Entity Registrant Name	FS BANCORP, INC.
Entity Incorporation, State or Country Code	WA
Entity Tax Identification Number	45-4585178
Entity Address, Address Line One	6920 220th Street SW
Entity Address, City or Town	Mountlake Terrace
Entity Address, State or Province	WA
Entity Address, Postal Zip Code	98043
City Area Code	425
Local Phone Number	771-5299
Title of 12(b) Security	Common Stock, par value $.01 per share
Trading Symbol	FSBW
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		4,246,041
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q2
Entity Central Index Key	0001530249
Amendment Flag	false