In a statement, the Central Election Commission (CEC) said that 94.5 per cent of votes cast in Sunday's referendum had backed the 40 constitutional changes, while only 3.3 per cent were against.
Turnout stood in the former Soviet state stood at 92 percent, or just over 4 million people, the CEC said.
As well as lifting the term limit for Rakhmon, the amendments also lower the minimum age for presidential candidates from 35 to 30, and ban the formation of religion- based parties.
But residents voting on Sunday in the near million-strong Tajik capital of Dushanbe appeared enthusiastic in their support for Rakhmon, who led the country out of a five-year civil war that began in 1992, less than a year after independence.
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"Rakhmon brought us peace, he ended the war, and he should rule the country for as long as he has the strength to," voter Nazir Saidzoda, 53, told AFP yesterday.
The lowering of the age limit for presidential candidates could position Rakhmon's 28-year-old son Rustam for an early succession, while restrictions on political parties come amid the ongoing trial of key members of a banned Islamic party.
The Islamic Renaissance Party of Tajikistan (IRPT) had been widely viewed as moderate before the government branded it a terrorist group last year, stripping away the most significant formal opposition to the Rakhmon regime.
Earlier in May, the autocrat signed off on a law creating a holiday in his honour proposed by parliamentarians in the two-chamber legislature which is completely loyal to his administration.
In February, the republic's youth affairs committee launched a contest for the best essays by schoolchildren in praise of the strongman's "heroic" rule.