"The TPP talks are critical to establishing strong economic footing for the United States in the Pacific region and providing a counterbalance to China.
"I look forward to consulting closely with Ambassador Kirk and his team as we continue our domestic process for Mexico to join TPP.”
The addition of Mexico officially ups the number of nations in the talks to 10. Canada also has been been invited, and that letter to Congress could come soon, possibly this week.
The nine nations — the United States, Australia, New Zealand, Chile, Peru, Singapore, Malaysia, Vietnam and Brunei — agreed in June to invite Mexico and Canada.
The countries are wrapping up their 13th round of talks in San Diego, with a final briefing set for Tuesday. Participants have said they want to conclude discussions either sometime this year or early next year.
A decision about Japan, the third nation that has pressed for entry into the talks, has yet to be made.
There is some hesitancy among lawmakers to bring that nation into the deal at this point, mainly because of what they say are restrictive policies that allow for very few U.S. auto exports to enter the country.
In Monday's letter, Kirk said Mexico is the second most important destination for U.S. exports, which increased 21 percent last year.
Kirk said Mexico's participation "will ensure that we have a cutting-edge agreement in place between us that will further enhance our trade relationship and benefit U.S. workers, manufacturers, service supplies, farmers, ranchers, small businesses and consumers."
"Mexico has assured us of its willingness to negotiate on these issues and its preparedness to achieve these high standards together with other TPP countries," he said.