The region sealed a semi-final shot against Ulster in Ravenhill next Friday after their Welsh rivals the Ospreys went down 37-19 in Leinster.
Treviso were celebrating their first double over Scarlets with tries for No 8 Robert Barbieri, flanker Monoa Vosawai, replacement centre Luca Mosari and wing Christian Loamanu. Outside-half Kris Burton kicked 21 points.
Scarlets joined Ulster, Leinster and Glasgow in the play-offs, with their places confirmed weeks previously.
Ulster finish the regular season atop the standings after they beat Cardiff Blues 37-19. Leinster finish second after their win over Ospreys, but both had been confirmed some weeks ago.
Ulster earned a home semi-final against the Scarlets thanks to two second-half tries from Andrew Trimble and a late score for 100-cap man Robbie Diack. Mark Anscombe's side had trailed 13-10 at half-time against a Blues effort which petered out in the second half.
Ruan Pienaar ended the night scoring 22 of Ulster's total with a first half try along with four conversions and penalties on a night when Chris Henry also played for the 100th time for his club..
Glasgow missed out on a home semi-final but tries from DTH van der Merwe and British and Irish Lions call-up Sean Maitland did see the Scottish side advance to the play-offs on the back of a 20-3 win over Connacht.
On a windy night in the west of Ireland, Connacht were hoping to provide a winning send-off for departing head coach Eric Elwood with the match also marking the final outings for the province of Johnny O'Connor, Adrian Flavin and the Leinster-bound Mike McCarthy.
However, a Ruaridh Jackson penalty was added by a Van der Merwe try as the visitors established a 10-3 half-time advantage with Dan Parks nailing a late penalty for Connacht.
Elwood's charges battled bravely on the restart with the wind at their backs, but the benefit of a strong replacements bench was vital for Glasgow as five points from the boot of another Lion, Stuart Hogg, allied to Maitland's 70th-minute effort, eased them over the line.
Edinburgh posted only their seventh win of the season when they saw off Dragons 31-24 at Murrayfield in the final match of their campaign.
Edinburgh had looked to be cruising when they entered the final quarter with a 31-10 lead after tries from Greig Tonks, Greig Laidlaw and Netani Talei, with Laidlaw kicking the rest of the points.
However, Dragons, who had led through an early try from Steffan Jones, profited from some poor defending and a wayward pass that produced late tries for Dan Lydiate and Tom Prydie, who contributed the rest of his side's points with the boot.
The defeat leaves Dragons second from bottom in the league table - one place behind Edinburgh and with only Zebre below them.