Saints ended County's hopes of retaining the Challenge Cup with a comfortable victory at Victoria Park.
Kenny Deuchar scored his first goal for Saints with a deflected shot in the 14th minute after the home side's defence failed to clear their lines.
County spurned two great chances to equalise through Stewart Petrie and Martin Scott just after half-time.
Saints secured the win in 49 minutes when Andy Jackson headed home Gary Irvine's cross.
Ross County: Cartis, McCulloch, Golabek (Iain Anderson 77), Dowie, Keddie, Lawson, Scott (Higgins 70), Adams, Barrowman, Petrie (Shields 77), Brady. Subs Not Used: Bell, Moore.
Booked: McCulloch, Iain Anderson.
St Johnstone: Cuthbert, Irvine, Dyer, Anderson, McManus, McInnes, Weatherston, McCluskey, Deuchar, Jackson, Sheerin. Subs Not Used: Main, MacDonald, Daal, Lawrie, Stewart.
Booked: Cuthbert.
Goals: Deuchar 14, Jackson 49.
Att: 1,100
Ref: I Brines
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