The former physics professor, known as “the German” on account of his ancestry, has used the limited powers of the presidency to back anti-corruption protesters and boost ties with the United States.
But critics say he shirks debates and lacks charisma.
He is expected to prevail over the candidate of the dominant Social Democrat Party (PSD), just as he did in 2014.
This time he is pitted against Viorica Dancila — a former prime minister with whom he has repeatedly clashed.
Praised by Trump
Iohannis’s first five years in office were marked by constant disagreement with the PSD, which has dominated politics since the communist era ended in 1989.
The 60-year-old has regularly rejected PSD initiatives, most notably a move to decriminalise several corruption offences.
Iohannis sided with hundreds of thousands of protesters — and the European Union — in opposition to those reforms.
In the lead-up to Sunday’s vote, Iohannis promised to rid Romania’s institutions of corruption — with the help of a new government spearheaded by the party he once led, the centre-right National Liberal Party (PNL).
“We have to keep the PSD far from power for years to come… to build a normal Romania where everyone feels at home,” he told supporters in a final rally Friday.
Away from domestic politics, Iohannis has been an ally of American President Donald Trump, twice visiting the White House and being praised by the US leader as someone who “could solve the corruption problem in Romania”.
“Iohannis was a corridor of communication with our partners… at a time when Romania’s affiliations to Western values were in doubt,” former foreign minister Cristian Diaconescu told AFP.
But even Iohannis’ supporters have sometimes complained that he has not been forceful enough — particularly when he buckled under pressure and allowed an anti-graft prosecutor to be dismissed.
Plea to emigrants
Often criticised for lacking charisma and being a poor public speaker, he refused to give in-depth interviews throughout his first term.
Former diplomat Ciprian Mihali told AFP Iohannis was a “cold man, calculated, who measures his every step”.
However, he faced a barrage of criticism in the run-up to Sunday’s election for refusing to join a debate with his opponent Dancila.
“I cannot accept a debate with a person who represents the worst in Romania’s politics from the last 30 years,” Iohannis said.
He later held a press conference and a Q&A; session but otherwise stuck to campaign rallies where he was always surrounded by his PNL allies.
Iohannis has faced embarrassment at the hands of the media before, particularly over questions about his personal wealth — he once owned six houses.
Before he became president, an interviewer asked why other teachers could not afford six houses and he replied it was “bad luck” — he faced severe criticism at the time and the media have continued to raise the issue throughout his tenure.
Iohannis’s career before the presidency was dominated by the 14 years he spent as mayor of Sibiu — his home city in central Romania, once host to a sizable German minority.
However, with the fall of communism and the 2007 EU accession, many Romanians with German roots chose to move to Germany — Iohannis’s parents among them.
The German population were not the only ones to leave — roughly four million Romanians live outside the country, many seeking better paid jobs in the European Union.
Despite his own family background, Iohannis has urged his countrymen to return, telling them: “I am Romanian… Me, I choose to stay, and I’m proud of it.”