The former Northampton Town forward, 25 has enjoyed three prolificseasons with Boro.
He will join the Lions in the summer regardless of what division they are in. They take on Scunthorpe in the League One play-off final next Sunday.
The Stevenage chairman Phil Wallace told the club's official website: "In my opinion Steve will adjust to whatever league he's in with Millwall next season and kick on again.
"He's a confident young man and the Championship will not faze him one bit if Millwall win at Wembley."
The Oldham Athletic midfielder Mark Allott will leave the club after rejecting the offer of a new contract.
Allott, 31, had been offered fresh terms by the new Latics manager Dave Penney, but feels that his future lies elsewhere.