Sonia Bompastor's side finished third in the Women's Super League last season, which means Chelsea must win this qualifying tie to reach the league phase of Europe's top competition.
Chelsea will host the first leg on August 26, with the return fixture on September 2. Kick-off times will be announced shortly.
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Real Sociedad finished third in Liga F last season, six points behind second-placed Real Madrid and 21 points adrift of league winners Barcelona.
Chelsea were seeded in the draw alongside Juventus, Paris Saint-Germain, Real Madrid and Wolfsburg.
There are nine qualifying ties in total and the winners of those two-legged matches will join the nine clubs who have directly qualified for the league phase.
The nine losing teams will drop into the Women's Europa Cup second qualifying round.
Women's Champions League third qualifying round ties
Sparta Prague vs Servette
PSV Eindhoven vs HB Koge
Brann vs Austria Vienna
OH Leuven vs Czarni Sosnowiec
Eintracht Frankfurt vs Paris Saint-Germain
St Polten vs Juventus
Wolfsburg vs Inter Milan
Chelsea vs Real Sociedad
Ajax vs Real Madrid
