Chelsea have won the UEFA Super Cup once, in 1998. In that match they beat Real Madrid.
At that time neither team had won the Super Cup, though Chelsea held the Cup Winners' Cup and their opponents Real Madrid had won the Champions League.
They played at the Louis II Stadium (Stade Louis II) in Moncao, a new venue at the time. The match was held on 28 August and for most of the time it looked like a draw. Then Gus Poyet scored a goal for Chelsea late in the match, giving them a 1-0 victory.
In 2012 it looked as if Chelsea might be going to win the cup again. However, although they got to the final they were defeated by Atletico Madrid, who beat them 4-0. This match was played at the same venue, but in fact it was the last time this venue will be used for the Super Cup.
You can read about Chelsea's 1998 win here.