A class 2 warning means that temperatures have been over 30C for five days in a row, or are expected to reach 33C for three consecutive days.
The heightened warning applies to large parts of Svealand and the entire area of Norrlandskusten in northern Sweden. Regions affected include Stockholm, Uppsala, Örebro, Värmland, Västmanland, Södermanland, and Dalarna.
Multiple cities experienced record-breaking temperatures yesterday, with Umeå reaching 32.2C - the hottest the city has been since 1882.
Other cities experienced the warmest temperatures since 1994, including Gävle and Sundsvall.
SMHI warned that the heat may shatter further records during Thursday.
"We have yet another warm day ahead of hus," Emil Björck, meteorologist at SMHI, told news agency TT. "I would not be surprised if the list of records is just as long tomorrow."