Senate Clerk Jeff Gossett said early Thursday morning the governor did not send him his veto, meaning either Sanford signed the bill or let it become law without his signature.
The governor's office did not return a phone message early Thursday morning to say what Sanford decided to do with the bill or explain his decision.
Sanford, who has scheduled a news conference for 10:45 a.m. Thursday, had said he planned to veto the bill because its penalties were not harsh enough.
The law allows officers to stop drivers who are not wearing a seat belt. Currently, adult drivers can be ticketed for failing to buckle up only if they are stopped for another traffic offense.
The compromise sets a $25 fine for seat belt offenses.
Sanford had said he wanted a bill that allowed a conviction for failing to buckle up to be a factor in injury lawsuits as well as causing repeat offenders to pay more for car insurance.