Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2025	Nov. 13, 2025
Document and Entity Information [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Transition Report	false
Document Period End Date	Sep. 30, 2025
Entity File Number	000-56790
Entity Registrant Name	Seneca Bancorp, Inc.
Entity Incorporation, State or Country Code	MD
Entity Tax Identification Number	39-4029114
Entity Address, Address Line One	35 Oswego Street
Entity Address, City or Town	Baldwinsville
Entity Address State Or Province	NY
Entity Address, Postal Zip Code	13027
City Area Code	315
Local Phone Number	638-0233
Title of 12(b) Security	None
No Trading Symbol Flag	true
Security Exchange Name	NONE
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		1,790,151
Entity Central Index Key	0002072421
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2025
Document Fiscal Period Focus	Q3
Amendment Flag	false