The party, called Unia Centrum (Centre Union), was unveiled at a press conference in Warsaw on Sunday.
It will operate under the UED's existing statute, with Hennig-Kloska taking over as leader.
The Centrum parliamentary caucus formed in mid-February after a split within the Polska 2050 (Poland 2050) party.
Hennig-Kloska lost a leadership contest there to Katarzyna Pełczyńska-Nałęcz and left with a group of MPs, later registering the Centrum Polska association.
The UED, her new merger partner, was previously chaired by Elżbieta Bińczycka, who becomes a deputy leader of the new grouping.
Hennig-Kloska described Unia Centrum as pro-business, pro-environment and pro-European, drawing on the values behind Poland's transition to democracy after 1989.
Almost all MPs from the Centrum caucus — 15 MPs and three senators — have signed up, she said, though four remain undecided.
MEP Michał Kobosko, who represents the Centrum group in the European Parliament, said the party aims to win voters who want a centrist Poland — one that supports business and the economy while taking climate change seriously.
On strategy, Hennig-Kloska said the plan was to contest next year's parliamentary election independently, while staying open to cooperation.
She was doubtful about a joint list with Polska 2050, seeing "no sign of a shift in its approach."
(ał)
Source: PAP