Muller will lead the party to the election in September, with Auckland Central MP Nikki Kaye as his deputy.
Members of the National Party caucus voted for the leadership change at an emergency meeting on Friday.
It came after a number of disastrous poll results for Bridges, including a Newshub/Reid Research poll released this week that put National’s support at just 30.6 per cent.
Muller confirmed his formal leadership challenge against Bridges in an email to his colleagues on Wednesday.
The National Party has released a statement following the caucus meeting confirming Muller’s new position as leader.
“There is no Team Todd, there is no Team Nikki, or anyone else – there is only Team National,” Muller said in the statement.
“National has always been a coalition of city and country, business and community, conservatives and liberals – National is the party for all New Zealanders.
“New Zealanders need a National government with the experience and management skills to get our country through the worst crisis since the end of the Second World War.
“My focus as leader is our country’s economic recovery and the strengthening of every community throughout New Zealand.”
