Scarlets was only pipped to top spot in Conference B on points difference by Leinster, but ensured it will join the Irish side in the last four by defeating Cheetahs 43-8.
Cheetahs was completely outclassed and did not score its first try until the 76th minute, having fallen behind early and never recovered.
Steff Evans crossed twice in the first half, while Leigh Halfpenny went over and a further three home tries followed after the break.
Munster is also through to the semi-finals after edging past Edinburgh 20-16, with the departing Simon Zebo creating a key try for Keith Earls.
Earls' score added to an earlier Rhys Marshall try, but Munster had to endure a nervy second half after Nathan Fowles brought Edinburgh back into contention 22 minutes from time.
It will face Leinster in a fortnight, while Scarlets travels to Glasgow Warriors a day earlier.