It took a great Shane Williams break from his own 22 to ease Ospreys clear, Hook supporting for the key try.
He added 14 points with kicks, Stephen Jones’ two first-half penalties the only scores for deflated Scarlets.
The Ospreys trail league leaders Leinster by five points, but they have a game in hand over the Lions who travel to Liberty Stadium on Friday.
The Scarlets gifted two early penalties to Hook.
Jones got the Llanelli side on the scoreboard with a penalty, and his second penalty got his team level, but Hook’s third kick restored the visitors’ advantage against the run of play.
On the stroke of half-time, Ospreys number eight Filo Tiatia was sin-binned but Jones missed the resultant penalty.
After the break the Scarlets were lucky not to have Dafydd Jones sent to the bin after a stamp at a ruck.
But a moment of genius from Williams broke the gloom at Stradey Park, the Ospreys wing bursting onto turnover ball inside his own 22. The Wales star stretched outside three tackles before reaching halfway and breaking right.
Williams then found support to the left, Sonny Parker being brought down short as he went for the line, but finding Hook from out of the tackle who crossed for a memorable score.
Brent Cockbain hobbled off with a dead leg that is likely to rule him out of the Leinster game, but Hook completed his scoring spree with a 63rd-minute drop goal.
Pens: S. Jones 2.
B. Davies, M. Jones, King, G. Evans, Watkins, S. Jones, Peel, I. Thomas, Rees, Dunlea, MacLeod, A. Jones, Easterby, G. Thomas, D. Jones.
L. Davies for Peel (80), Manu for Dunlea (49), Afeaki for A. Jones (49), Popham for D. Jones (49).
Tries: Hook. Cons: Hook. Pens: Hook 3. Drop Goals: Hook.
Byrne, Walker, Parker, A. Bishop, S. Williams, Hook, Marshall, D. Jones, B. Williams, Millward, Cockbain, Thomas, R. Jones, Pugh, Tiatia.
Terblanche for Walker (80), Spice for Marshall (79), Hibbard for B. Williams (69), Griffiths for Millward (69), Powell for Cockbain (59).
Hugh Watkins (Wales).
Bristol 30 Leicester 13.