Oxford West and Abingdon MP Layla Moran will be standing in the contest to become the leader of the Liberal Democrats.
She said her party faced an “existential challenge” and needed to focus on a positive vision for the UK.
Jo Swinson resigned after losing her seat at the general election in December, in which the Lib Dems dropped from 12 to 11 seats.
Nominations for candidates will open on 11 May, the party has said.
Last month, Bath MP Wera Hobhouse announced she was entering the race.
Declaring her candidacy, Ms Moran said the decision had been a “long-time coming”, but she had felt “frustrated” over the last three years about the direction the party was going in.
She said: “We have spoken a lot about what Liberal Democrats at a national level are against, but that’s just not good enough and people want to have a positive vision for the country.”
The 37-year-old said she wanted to make sure a no-deal Brexit was avoided, and she would prioritise opportunity, education and climate change, according to the Local Democracy Reporting Service (LDRS).
She said she wanted to promote “pragmatic” cross-party work, and “kinder gentler politics”.
Ms Moran, who is currently the Lib Dem spokesperson for education, has been an MP since 2017.
In January, the former physics teacher announced she was pansexual and in a relationship with a woman.
Nominations for the contest will close on 28 May, before the ballot will opens on 18 June and concludes on 15 July.
The party says it has more than 100,000 members who will be eligible to take part in the selection process.
Ex-cabinet minister Sir Ed Davey and party president Mark Pack are currently joint acting leaders of the party until the election process is completed.