Andrew Mwangura of the Mombasa-based East African Seafarers' Assistance Programme said his group was seeking more details on the vessel, which he named as the Laxmi Sagar.
"It was taken, I think, yesterday. The crew is Indian ... we don't know how many they are, but the dhows have always got more than 10 people (on board)," said Mwangura.
"We are in touch with the Indian ship registry. We don't know for sure where she was taken. That comes to the third dhow that has been taken in the span of seven days."
Heavily armed Somali pirates have made tens of millions of dollars in ransoms by hijacking ships in the Indian Ocean and strategic Gulf of Aden, which links Europe to Asia.
Last week, the Greek ship Ariana and its 24 Ukrainian crew members were freed after a helicopter dropped a multi-million dollar ransom onto its deck.
Anti-piracy naval operations by the European Union, NATO and several individual states have failed to stop the pirates, who still hold 11 ships and some 280 crew. The multi-national patrols have only appeared to push the gangs to hunt further from shore.
On Sunday, freshly trained Somali marine forces said they had captured three suspected pirates in a speedboat off Mogadishu with the help of African Union peacekeepers.
Experts say efforts to set up an international court for pirates are hampered by the complex laws of the sea, national sovereignty and lack of effective police.