Iohannis, 60, an ethnic German whose promise to rein in corruption has helped make him Europe's most popular political leader on Facebook, had been named by diplomats as a possible replacement to European Council head Donald Tusk.
The country's fourth president since the 1989 fall of communism, he inflicted a shock defeat on then premier Victor Ponta in 2014, on a promise to fight corruption in the EU's most graft-prone member and be a unifying leader.
"...There are important people, both in the country and in Europe, who would see me in a leading position in a European institution," Iohannis told a gathering of ethnic German members of the FDGR party in his hometown of Sibiu.
"I want to say it here now publicly: I am the president of Romania and I want to remain the president of Romania and not of the European Council, because there is much more to do here and I am willing to remain involved."
As Iohannis ranks first with about 43% in opinion polls to win a fresh term, far ahead of any other politician, his departure would have left his main ally, the centrist opposition National Liberal Party scrambling to find a suitable candidate to run in the November presidential ballot.
A former high-school physics teacher, Iohannis was praised for his managerial skills in running the Transylvanian town of Sibiu. A descendant of Saxons who settled there in medieval times, he is Romania's first president from an ethnic minority.
Iohannis is a staunch critic of the ruling Social Democrat's (PSD) drive to sap the judiciary. Since taking power in late 2016, the PSD has chipped away at the independence of the judiciary, prompting criticism from the EU.
Voters punished the PSD in a May 26 European Parliament election, which the party lost to centrist groupings.
Romanians also overwhelmingly endorsed a non-binding referendum called by Iohannis to prevent the government from further changing legislation via emergency decree and from granting pardons and amnesty for graft convictions.
PSD's Prime Minister Viorica Dancila is expected to cement her leadership at the helm of the ruling party at a congress on Saturday. Her party has yet to nominate its choice for Romania's presidential ballot.
(Editing by Stephen Powell)
