The 27-year-old joins on a free transfer from the Scottish Premiership outfit and bolsters the Glovers’ options up front.
His first outing in green and white could come against Manchester United in the Emirates FA Cup fourth round tonight.
Manager Darren Way said: “It’s good signing for us. We tried to get him in the summer and I nearly divorced wife with the amount of time I spent on the phone to him!
“We couldn’t quite match Motherwell at the time. With his age he’s got experience and can help the younger players.
“Having played in the SPL dropping down to us in League Two is huge.”
London-born Fisher joined Motherwell after scoring eight goals in nine matches for Inverness Caledonian Thistle at the end of the 2016/17 SPL season.
A product of Oxford United’s youth setup, the forward left the U’s to become part of the Glenn Hoddle Academy and through the system signed for Jerez Industrial.
He moved on to the Belgian Second Division with KVK Tienen before further spells in Belgium with KRC Mechelen and KSK Heist.
Following a short stint in Italy with AC Monza Brianza 1912, Fisher returned to England, signing for Mansfield Town.
Sixteen appearances later, the big frontman was snapped up by Torquay United and his impressive performances led to a move north of the border.
Fisher has featured 19 times for Motherwell this season and will now ply his trade in Somerset until the end of the 2018/19 campaign.