Bangladesh's donors should press the government to release Khan and end the harassment of human rights defenders and other critics of the government, the rights body said.
Khan, a lawyer and director of Odhikar, one of Bangladesh's leading human rights organisations, was arrested in front of his family by approximately 10 plainclothes members of the Detective Branch.
At a hearing Sunday, the court denied bail for Khan without showing that he was a flight risk, and ordered Khan to be held for five days before the next hearing.
Khan has long expressed concern that he would be arrested for political reasons but said that he wanted to stay in the country to do his work despite the risks.
Human Rights Watch called for Khan to have regular access to counsel while in custody.
"The arrest of Adil Khan appears to be an attempt to silence one of the Bangladeshi government's most vocal critics prior to national elections," said Brad Adams, Asia director at Human Rights Watch.
"The Awami League government should not respond to its low public approval ratings by making a mockery of its claims to respect the free speech rights of everyone."