Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2021	Nov. 01, 2021
Document Information [Line Items]
Document Type	10-Q
Document Quarterly Report	true
Document Transition Report	false
Document Period End Date	Sep. 30, 2021
Entity File Number	001-38916
Entity Registrant Name	Bicycle Therapeutics plc
Entity Incorporation, State or Country Code	X0
Entity Tax Identification Number	00-0000000
Entity Address, Address Line One	B900
Entity Address, Address Line Two	Babraham Research Campus
Entity Address, City or Town	Cambridge
Entity Address, Country	GB
Entity Address, Postal Zip Code	CB22 3AT
City Area Code	+44
Local Phone Number	1223 261503
Entity Filer Category	Non-accelerated Filer
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		29,399,709
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
Entity Central Index Key	0001761612
Amendment Flag	false
Entity Ex Transition Period	true
Ordinary Shares
Document Information [Line Items]
Title of 12(b) Security	Ordinary shares, nominal value £0.01 per share *
No Trading Symbol Flag	true
Security Exchange Name	NASDAQ
American Depositary Shares
Document Information [Line Items]
Title of 12(b) Security	American Depositary Shares, each representing one ordinary share, nominal value £0.01 per share
Trading Symbol	BCYC
Security Exchange Name	NASDAQ