In a letter written to Delhi Police Commissioner B S Bassi on November 12 last year, he had said that "such conduct" by one of the Delhi Police officer against his domestic help Narayan Singh was completely unacceptable and illegal.
Tharoor said he and his staff had "cooperated fully" with the police probing the case.
"I was therefore shocked and appalled to learn that in the course of the 16-hour interrogation conducted by 4 Delhi Police officers on Friday (7/11/14) and again during the 14-hour interrogation on Saturday (8/11/14), my domestic helper Narayan Singh was repeatedly physically assaulted by one of your officers.
"Worse, the officer used the traumatic physical assault to try and intimidate Narayan into 'confessing' that he and I murdered my wife," Tharoor said in the letter.
Tharoor also referred to his telephonic conversation with Bassi on November 8 when he had expressed his concerns about the alleged police action.
"As you graciously agreed, such conduct is completely unacceptable and illegal. It also amounts to the use of physical coercion in the attempt to frame an innocent man. I would request you to take immediate and appropriate action against such unlawful misconduct of the officer concerned," he said.
Delhi Police had on Tuesday registered a murder case into Sunanda's death under section 302 of the IPC on the basis of an AIIMS medical report.
The report had concluded that her death was unnatural and due to poisoning, but nobody has been named as a suspect as yet. The FIR was filed against unknown persons.
Sunanda, 51, was found dead in a five-star hotel in the national capital on January 17 last year.