Solis sent a letter to colleagues Wednesday announcing her resignation.
In it, she gave no specific reason for why she's leaving.
She called the decision to resign "one of the most difficult decisions" she'd ever made.
Other top Obama administration officials are also leaving their posts after one-term, including Secretary of State Hillary Clinton.
President Obama released a statement calling Solis a "champion for working families" and commending her work over the past four years.