Sligo captain Darragh Cummins kicked four points from play in the second-half as they reached their first Connacht U-21 final since 2012.
Sligo are the only county in the province not to have won a title at this grade and they will now go to face the winners of Galway and Mayo in the decider.
After a tough 30 minutes playing into the wind Sligo were 0-6 to 0-4 behind at half-time, but Paddy O'Connor kept them in touch.
And after the restart Cummins turned things around with a storming display from midfield, while Jodie O'Reilly hit the crucial goal in the 47th minute that proved the difference.
John Gannon scored Roscommon's first point in the third minute, before O'Connor hit back. Roscommon took over half-way through the first-half, and scored four points in a row.
Adrian Dockery's men had got the better of the Sligo U-21s in three Connacht semi-finals on the trot. And with points from Hubert Darcy (two), Jamie Fahy and Cian McKeon they took a 0-6 to 0-3 lead.
But O'Connor scored his third point of the half and Sligo trailed by two at half-time. Sligo took over afterwards with Cummins hitting four points, and O'Reilly sealing it.
Scorers - Sligo: J O'Reilly 1-1, P O'Connor 0-4 (4f), D Cummins 0-4 each, L Gaughan, S Connolly, J Perry, S Carrbine 0-1 each. Roscommon: H Darcy 0-5 (5f), C McKeon 0-4 (3f), S Mannion 0-1 (f), J Gannon, J Fahy, B Kelly each.
Sligo - D Cafferkey; E McGuinness, L Nicholson, M Gordon; G O'Kelly-Lynch, P McNamara, S Power; D Cummins, P Kilcoyne; A McLoughlin, L Gaughan, S Connolly; K Cawley, P O'Connor, J O'Reilly. Subs: S Carrbine for Connolly (37), J Perry for McLoughlin (46), N Mullen for McGuinness (55), F Cawley for Kilcoyne (60), K Banks for O'Reilly (65), M Clarke for Perry (67, black card).
Roscommon - S Mannion; J Connaughton, K Beirne, M Richardson; S Conroy, A Lyons, L Daly; G Keenan, H Darcy; B Stack, J Gannon, C Payne; B Kelly, C McKeon, J Fahy. Subs: J Fetherston for Keenan (37), C Callinan for Fahy (38), A Brogan for Gannon (44), T Butler for Payne (49), D Connaughton for Daly (52).
REF - Jerome Henry (Mayo).