Chantal Ramazani, who is now in hiding, believes the intention was to block Mr Katumbi's bid for the presidency in elections due in November.
President Joseph Kabila is required by the constitution to step down this year but no poll date has been set.
The intelligence services deny the accusations.
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Last month, Mr Katumbi was convicted of illegal property selling and sentenced to three years in prison, in absentia.
The 51-year-old businessman, owns one of Africa's biggest football clubs, is currently out of the Democratic Republic of Congo (DRC) seeking medical treatment.
In a letter addressed to the president and other politicians and organisations, Judge Ramazani says the facts against Mr Katumbi were never examined in tribunal.
She claims the purpose of the trial was cripple Mr Katumbi's political future and that she was threatened with losing her job, even imprisonment, if he were not convicted.
After sending the letter, she went into hiding.
The BBC's Maud Jullien, in the Congolese capital Kinshasa, says Mr Katumbi is seen as a favourite in the race for the presidency and is expected to return to the country before an opposition rally on Sunday.
Opponents of President Kabila say he is trying to stall elections to stay in power, but the constitution states he must stand down after his two terms in November.
Moise Katumbi was governor of the south-eastern Katanga province for almost a decade.
In September last year he broke ties with the ruling party when he accused President Kabila, his former ally, of wanting to cling to power.
His popularity is partly down to his job as the president of a great source of Congolese pride - football club TP Mazembe.
They are Africa's reigning football champions, having won the African Champions League for the fifth time in November.
A judge in the Democratic Republic of Congo has said she was pressured by the intelligence service to convict opposition politician Moise Katumbi.