"I have social sensitivity and I want to do more in this area, and together we can do more," Brglez said as he made the move official side by side with Matjaž Han, the head of the SD deputy faction.
"It's the result of cogent consideration and realisation that together we can do more for the values of social democracy in Slovenia and thus more for the people," Han said.
He added that in the previous term of parliament, when Brglez served as the speaker, they often shared the same views, values and principles.
"This transfer is substantive one rather than political. There's no conspiracy theory, it's that with Brglez we get an excellent MP, expert and a person who can do a lot for social democracy, the country and shares our views," Han said.
No more details were given, but it appears that Brglez will not be joining the SD party for the time being, while this is not ruled out in the future.
Brglez wished all the very best to his old deputy faction and called for constructive cooperation. "After all, today it's time for the left wing and centre left to unite rather than drift apart."
Brglez's defection does not upset the balance of power in parliament on a larger scale because the SD and the SMC will both make part of a coalition headed by the Marjan Šarec List (LMŠ), along with the Pensioners' Party and Alenka Bratušek Party.
Brglez, who was the vice-president of the SMC, was expelled from the party on 26 June, three weeks after the general election.
The decision was taken amid coalition talks as Brglez publicly announced that he was willing to stay on as the speaker, while the party wanted the post be taken by party leader Miro Cerar.
Cerar explained the decision to expel his former close aide by saying that Brglez had "caused a lot of damage to our party through his explicit disrespect for decisions taken by the party's bodies".
Brglez argued that his expulsion was orchestrated and based on false pretences, indicating that the real reason was his offering himself for the post of speaker, and his intention to challenge Cerar for the leadership of the party.
Cerar, the outgoing prime minister, established the SMC, which was initially named the Miro Cerar Party, ahead of the 2014 general election and was confirmed for a second term unanimously at the party's congress in March this year.