Another 11 people were wounded when the attacker blew himself up inside Saleh Mohammad Asikzai's campaign office in the southern city of Lashkar Gah, Helmand provincial governor spokesman Omar Zhwak told AFP.
Asikzai, a young first-time candidate, was campaigning on a platform of "positive change".
There was no immediate claim of responsibility for the attack. Helmand has long been a stronghold for the Taliban, which was toppled from power in a US-led invasion in 2001.
It is not clear how many people were inside the room at the time of the blast, which comes a day after the Taliban warned candidates to pull out of the "bogus" election scheduled for October 20.
It was the second suicide attack to target a parliamentary candidate since campaigning officially kicked off on September 28.
An attack on a rally in the eastern province of Nangarhar on October 2 killed 13 people and wounded more than 40.
The Islamic State group claimed the attack, which the candidate survived.
Violence had been expected to escalate ahead of the poll, which more than 2,500 candidates are contesting.
Tuesday's attack takes the number of candidates murdered in targeted killings to at least six.
Hundreds of civilians also have been killed or wounded in poll-related violence in recent months.
Preparations for the ballot, which is is a test run for next year's presidential vote, have been in turmoil for months and there has been debate about whether the vote should go ahead.
Bureaucratic inefficiency, allegations of industrial-scale fraud and an eleventh-hour pledge for biometric verification of voters threaten to derail the process, which is three years late.
Some 54,000 members of Afghanistan's beleaguered security forces will be responsible for protecting more than 5,000 polling centres on election day.
But there are concerns over how they will manage as the Taliban and IS step up attacks across the country.
At least 12 soldiers and 30 Taliban fighters were killed in the attack, he said.
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