Veerendra Kumar, the chairman and managing director of the Malayalam daily newspaper Mathrubhumi, is also a director in PTI.
'I have taken a decision to resign from Rajya Sabha.
I do not wish to continue as RS member under Mr Nitish
Kumar who joined sangh parivar', he told reporters.
"I have already communicated the decision to Mr Nitish
Kumar", he said.
Asked when he will put in his papers, Kumar said 'I will let you (media) know'.
On his plans, Kumar said: "First we will discuss the party's future in the state committee and decide further course of action."
He said the Bihar Chief Minister told him not to resign and that he considers him as one of the tall socialist leaders in the country.
"But I have conveyed my decision to Mr Nitish
Kumar and party", he said.
JD(U) is presently a partner in the Congress-led UDF
Opposition in Kerala.
Kumar joined the UDF
fold after snapping ties with CPI-M led ruling LDF
over differences on Kozhikode Lok Sabha seat with CPI-M in 2009.
Referring to reports that his decision to resign was the first move to return to ruling CPI-M led LDF
fold in the state, Kumar said 'Now we are in UDF
and we have not discussed the issue with LDF'.
However, Kumar said he had held talks with some of the leaders of JD(S), a partner in the LDF, in the state and that they expressed desire to work unitedly.
Kumar, who was the leader of the Socialist Janata (Democratic) party, merged it with the Janata Dal (United) in 2014.
He was a minister in the United Front government after being elected to the Lok Sabha in 1996 on the Janata Dal ticket.
He was in Janata Dal (Secular) headed by former Prime Minister H D Deve Gowda from 1999 to 2010.