The 53-year-old will replace lawyer, Jeanette Chong-Aruldoss, who had been the NSP’s secretary-general since October 2013.
She contested the Mountbatten single-seat ward in the 2011 General Election and lost to Mr Lim Biow Chuan of the People’s Action Party (PAP), garnering 41.4 per cent of the vote.
It is understood she did not contest the position of secretary-general.
Business consultant Sebastian Teo, 66, was re-elected to his sixth consecutive term as party president.
Mr Tan, who is believed to have joined the party after the 2011 General Election was surprised to be elected.
He saidd, “Since I have taken up this heavy responsibility, I will give it my all.”
“We will continue to work hard on previous ground.”
The father of three has been active in social and counselling work and is also a museum docent.
The NSP fielded candidates in 24 seats in the 2011 GE, including four single-ward seats and four Group Representation Constituencies (GRC).
It was the third best performing party in the election, after the PAP and the Workers’ Party, but did not win any seats in Parliament.