The knockout stage of the 2004–05 UEFA Cup began on 16 February 2005, and concluded with the final at the Estádio José Alvalade in Lisbon on 18 May 2005.
The team that had the higher aggregate score over the two legs progressed to the next round.
If away goals are also equal, 30 minutes of extra time were played.
In the final, the tie was played over just one leg at a neutral venue.
If scores were level at the end of normal time in the final, extra time was played, followed by penalties if scores had remained tied.
2–2 on aggregate; Steaua București won 4–3 on penalties.
3–3 on aggregate; Austria Wien won on away goals.
The final was played on 18 May 2005 at the Estádio José Alvalade in Lisbon, Portugal.