A person familiar with the contents and those who received the messages, who spoke on condition of anonymity to discuss sensitive matters, confirmed the second chat to The Associated Press.
The second chat on Signal - which is a commercially available app not authorised to be used to communicate sensitive or classified national defence information - included 13 people, the person said. They also confirmed the chat was dubbed "Defense ' Team Huddle".
The New York Times reported that group included Hegseth's wife Jennifer, a former Fox News producer, and his brother Phil Hegseth who was hired at the Pentagon as a Department of Homeland Security liaison and senior adviser. Both have been travelling with the defence secretary and attending high-level meetings.
The revelation of the additional chat group brought fresh criticism against Hegseth and President Donald Trump's wider administration after it has failed to take action so far against the top national security officials who discussed plans for the military strike in Signal.
"The details keep coming out. We keep learning how Pete Hegseth put lives at risk. But Trump is still too weak to fire him," Senate Democratic Leader Chuck Schumer posted on X. "Pete Hegseth must be fired."
The first chat, set up by national security adviser Mike Waltz, included a number of Cabinet members and came to light because Jeffrey Goldberg, editor-in-chief of The Atlantic, was added to the group.
The contents of that chat, which The Atlantic published, shows that Hegseth listed weapons systems and a timeline for the attack on Iran-backed Houthis in Yemen last month.
The White House and the National Security Council did not respond to messages seeking comment about the additional chat group.
It comes as four officials in Hegseth's inner circle departed last week as the Pentagon conducts a widespread investigation for information leaks.
Dan Caldwell, a Hegseth aide, Colin Carroll, chief of staff to Deputy Defense Secretary Stephen Feinberg, and Darin Selnick, Hegseth's deputy chief of staff, were escorted out of the Pentagon.
While the three initially had been placed on leave pending the investigation, a joint statement shared by Caldwell on X on Saturday said the three "still have not been told what exactly we were investigated for, if there is still an active investigation, or if there was even a real investigation of 'leaks' to begin with".
Caldwell was the staff member designated as Hegseth's point person in the Signal chat with Trump cabinet members.
Former Pentagon spokesman John Ullyot also announced he was resigning last week, unrelated to the leaks. The Pentagon said, however, that Ullyot was asked to resign.
