Pat Clarke scored two goals in two minutes to earn Raith Rovers a final-day victory at Cappielow.
The visitors had won both of their last two games to preserve their First Division status and took the lead after 16 minutes thanks to Jamie Walker's first goal since early March.
Morton's best chance of the first half fell to Archie Campbell who volleyed over from 12 yards out six minutes before half-time.
Clarke doubled Rovers' advantage after 52 minutes when he latched on to John Baird's assist to slot the ball past Alan Combe.
The Ton's chances of getting back into the game ended two minutes later when the Dunfermline loanee nodded Walker's cross into the bottom corner to notch his second goal of the afternoon.
Michael Tidser crossed for Lewis Hawke to head Allan Moore's side's consolation 13 minutes from time.
Morton: Combe, Evans, Weatherson, Graham, O'Ware (Di Giacomo 62), Derek Young, Smyth, Bachirou, Tidser (Frizzell 86), MacDonald, Campbell (Hawke 73). Subs Not Used: McGinley, Orr.
Goal: Hawke 77.
Raith Rovers: McGurn, Jason Thomson, Hill (Vaughan 85), Murray, Dyer, Jamie Walker (Callachan 65), Allan Walker, Davidson, Hamill, Baird, Clarke (Casalinuovo 70). Subs Not Used: Laidlaw, Williamson.
Booked: Davidson, Hill, Clarke, Callachan.
Goals: Jamie Walker 16, Clarke 52,54.
Ref: Kevin Clancy