Erdogan co-founded the party in 2001 but was obliged to stand down from his role as AK Party leader when elected president in 2014.
"Today, I return to the party I founded", Erdogan said after rejoining the AKP at a ceremony held for official registration of the president at the party headquarters in capital Ankara with participation of hundreds of party officials.
If confirmed, it would be the first time that the president will be both party chairman and head of state since the end of the presidency of Ismet Inonu, the successor and right-hand-man of Mustafa Kemal Ataturk, Turkey's modern founder. Turkey's third president, Celal Bayar, was also a member of the Democrat Party.
Erdogan is expected to be re-elected as its chairman at a congress on May 21.More news: US Says Turkish Airstrikes In Syria, Iraq Kill Partner Counter-ISIS Fighters
Earlier it was not constitutional for the president to be directly affiliated to a political party.
Erdogan's "yes" camp won the referendum with 51.4 per cent of the vote against 48.6 per cent for the "no" side, according to final results announced last week.
The vote switches Turkey's system from a parliamentary one to a presidential system, abolishing the office of the prime minister while boosting the president's powers.
"There is no option other than opening chapters that you have not opened until now", Erdogan said, referring to the individual policy areas that need to be concluded before Turkey joins the EU.More news: Le Pen Takes Leave As FN Head In French Presidency Bid
After years of stalemate on Turkey's bid to join the world's biggest trading bloc, European Union governments say the process is dead, citing Erdogan's crackdown on dissidents, his "Nazi" jibes at Germany and a referendum giving him sweeping new powers that a rights group says lack checks and balances. "If you do not open them: Goodbye".
"You (Europe) will keep your promises". Then we will sit at the able and talk.
Supporters of the new system say it will bring Turkey efficient governance but opponents fear it will set the country on the path to authoritarian rule. "Otherwise, we have nothing left to discuss with you", Erdogan said. The change will allow him to lead the party outright.More news: Quake rattles Alaska, Canada border area: USGS