Polls opened on Monday morning but the outcome is unlikely to be a surprise - Tajikistan's main opposition parties are boycotting the elections.
Rakhmonov, who won the last election in 1999 with 96.4 per cent, has been criticised by both domestic opposition groups and foreign observers for undermining civil rights and jailing dissidents.
In Monday's elections he faces four obscure rivals from government-friendly parties.
The Central Asian country's three largest opposition parties, the Islamic Renaissance Party, the Democratic Party and the Social Democratic Party are all boycotting the election, saying that the vote will be neither free nor fair.
Formerly communist Tajikistan maintains close ties with Russia
The Organisation for Security and Cooperation in Europe (OSCE) has also criticised the elections, in which 3.2 million people are eligible to vote.
In a report on the elections released on Monday, its observers said: "No signs of a competitive campaign have been observed thus far."
"Candidates' platforms are similar; none of the four candidates running against the incumbent provides a political alternative to ... Rakhmonov's programme."
On Saturday, the opposition - fragmented and small - held a rare street protest against what they see as a Soviet-style show election.
Three activists were arrested after the rally and jailed for 15 days.
Memories of civil war
Tajikistan was wrecked by a civil war in the early 1990s in which more than 100,000 people died. The war was won by Rakhmonov's Russian-backed forces.
Despite the lack of democracy, Rakhmonov appears to be genuinely popular.
Most Tajikistanis polled by Reuters in Dushanbe and nearby towns said they believed he could ensure peace and stability in their impoverished nation.