Simon Mann was jailed in 2004 and was briefly released after serving his sentence in May last year. However, he was again arrested on an immigration warrant while awaiting deportation.
Mann, 55, was convicted of trying to buy weapons without a licence as part of a plot against Equatorial Guinea President Teodoro Obiang Nguema Mbasogo.
His lawyer, Jonathan Samkange, told Reuters on Wednesday he would appeal against Wednesday's ruling.
"We are appealing the first thing tomorrow (Thursday) but we have not had full sight of the whole judgment as it was only delivered this evening," Samkange said.
Mann was alleged to be at the centre of a plot against oil producer Equatorial Guinea, one of the continent's richest countries and one of its most repressive.
On Wednesday, the High Court dismissed Mann's argument that he would not receive a fair trial and would be tortured if he were deported to Equatorial Guinea.
Authorities in Equatorial Guinea authorities have assured Zimbabwe that Mann -- believed by his government to be the "intellectual head" of the coup plot -- would receive justice.
He was arrested in March 2004 when he met a plane carrying dozens of men and military equipment which landed in Harare on what officials said was the first stop on their way to launching a coup against Obiang.
Eleven other men, including several foreigners, are serving sentences of between 13 and 34 years in Equatorial Guinea in connection with the alleged plot.
Mark Thatcher, the son of former British Prime Minister Margaret Thatcher, was accused of helping to finance the coup attempt and admitted taking part. He avoided jail in a deal with prosecutors in South Africa, where he lived.