Westcarr, 34, played in the Football League for Nottingham Forest, Notts County, Chesterfield, Walsall and Portsmouth, as well as spells at several other clubs.
He scored 16 goals in the EVO-STIK Premier Division last season.
Westcarr said: "I've seen the other players that have come in and they've said the same.
“They've been sold what Paul and Russ want from their squad and I'm no different.
“Pre-season is going to be hard work, but it's what I want. We're going to start pre-season as a good group, looking to be at our best for the club and its supporters."
Returning to the Gingerbreads is striker Gregg Smith, who scored 32 goals in 89 appearances for Grantham before leaving in 2014.
He has since played for King’s Lynn, Tamworth, Boston United, Brackley Town and Stamford.
Midfielder James Baxendale, 26, has also joined Paul Rawden’s squad for next season.
He finished last season at Mickleover Sports, and previously made more than 100 appearances for Walsall in the Football League.
Meanwhile, defenders Tom Ward and Connor Bartle have both agreed to stay at the club next season.