Slay, a cornerback from Mississippi State, retweeted a report Thursday that he had signed a four-year, $5 million contract that includes a $2.9 million signing bonus.
The others who have signed include offensive guard Larry Warford (third round), receiver Corey Fuller (sixth round), running back Theo Riddick (sixth round), tight end Michael Williams (seventh round) and linebacker Brandon Hepburn (seventh round).
The three remaining picks who hadn't signed as of Thursday afternoon are defensive end Ziggy Ansah (first round), defensive end Devin Taylor (fourth round) and punter Sam Martin (fifth round).
The Lions' three-day rookie mini-camp begins Friday at their headquarters in Allen Park. Players who haven't signed can still participate by signing a waiver.
Slay, however, won't be available because he's recovering from arthroscopic knee surgery for a torn meniscus.