Russell, 28, was part of the side that won April's World Twenty20 and will be available for four matches in June.
Tahir, 37, will then link up for the final seven games and is set to play in six County Championship fixtures.
"Andre is one of the leading T20 players in the world right now," said director of cricket Mick Newell.
Russell has taken 180 wickets in T20 cricket at an average of 26.13, and has 12 half-centuries with the bat with a strike rate of more than 165 runs per 100 balls.
Newell added: "Looking at our line-up, there was certainly scope to add a hitter into the middle order, as we did in 2015 with Darren Sammy.
"In Andre's case it's an added bonus that he's a very effective fast bowler as well."
Tahir played three matches for Notts towards the end of last season.
He has taken 150 wickets in international and domestic T20 cricket at 20.82, with an economy rate below seven.
"We've seen many times how valuable a world-class leg-spinner can be, in all formats of the game," said Newell.
"Imran enjoyed his time here last year, he wanted to come back, and we're only too happy for him to do so.
"Having him available will also allow us to make the most of some of the drier pitches in four-day cricket at the back end of the summer."
Nottinghamshire have signed West Indies all-rounder Andre Russell and South Africa leg-spinner Imran Tahir for their T20 Blast campaign.