Chief Justice Godfrey Chidyausiku and four Supreme Court justices said an urgent appeal filed by expatriate Zimbabweans "has no merit and should be dismissed".
The ruling, announced on Thursday, was reported by state media on Friday.
The ruling, from a court led by an ally of President Robert Mugabe, is likely to increase questions about whether the 31 March vote can be free and fair.
Most of those abroad are thought to support the main opposition, Morgan Tsvangirai's Movement for Democratic Change, which was narrowly defeated in disputed elections in June 2000 and March 2002.
Detailed reasons for the ruling on expatriate voters will be released later by Chidyausiku, a former Cabinet minister installed by President Mugabe in 2001 after his predecessor, Anthony Gubbay, was forced to step down in the face of death threats.
It is estimated that four million
Zimbabweans live abroad
Happias Zhou, lawyer for the seven UK-based Zimbabweans who brought the test case, had argued their fundamental right to vote had been violated. Government lawyer Ernest Jena held that those abroad failed to meet residence qualifications.
Thursday's ruling stipulated that only diplomats and servicemen posted abroad will be allowed to vote, by mail. Opposition lawyers have charged that during past elections, soldiers on missions abroad were ordered to mark multiple ballots for the ruling party.
The opposition complains it has never been allowed full access to rolls containing the names of 5.6 million voters. It fears more than one million may be so-called ghost voters for the ruling party.
Members of the Zimbabwean diaspora last year boosted its failing foreign currency reserves by sending back more than $100 million through a state-sponsored Homelink scheme, and some are now calling for a boycott of the programme.
The Reserve Bank of Zimbabwe estimates the number of economic refugees at up to four million, with two million in neighbouring South Africa, 400,000 in Britain, and substantial numbers in Canada, the United States and Botswana.