The 28-year-old was a free agent after leaving Exeter's biggest rivals at the end of his contract in the summer.
Reid has also played for Yeovil Town, West Bromwich Albion, Brentford and Rotherham United.
He scored a hat-trick against Exeter in the Devon derby in 2015 - the first in the fixture since 1929 - and could make his debut for the Grecians against Argyle in League Two on Saturday.
Reid scored 21 goals for Argyle during the 2013-14 season while on loan at Home Park and 20 the season after.
But last term he managed just seven goals as he was hampered by a foot injury, later telling the BBC that his season had been a 'write off'.
Exeter need more firepower, having scored just six goals in the league so far this season, with half of those coming in their 3-2 victory at Colchester earlier this month.
"I have been in talks with a few teams, but I met the manager (Paul Tisdale) on Sunday in a nice pub in Bath and he explained what he wanted to do for me," said Reid
"I went home feeling refreshed and it left me with a massive smile on my face. I just couldn't wait to get the deal over the line."
"I played against his teams for years now throughout my career and Exeter, whether they have been in the division above or League Two, have always been good on the eye to watch," he added.
Exeter City have signed former Plymouth Argyle top scorer Reuben Reid.