Tharoor, who was asked on Wednesday to join the probe by the special investigative team (SIT) investigating Pushkar's death, is being interrogated at south Delhi's Sarojini Nagar Police station.
The SIT team had earlier questioned him on Jan 19 at its Anti Auto Theft Squad (AATS) office in south Delhi's Vasant Vihar.
Delhi Police Commissioner B.S. Bassi had said on Wednesday that Tharoor will be questioned again on Thursday.
"After questioning Pushkar's son Shiv Menon and other people, we need to question Tharoor again. He has been asked to appear before the SIT tomorrow," Bassi had said.
Police had constituted an SIT to probe the murder after registering a murder case against unknown people Jan 1 this year.
Sunanda Pushkar was found dead in a luxury hotel room Jan 17, 2014.