"After all these years of the Obama-Clinton failures, it's time we elect a strong new leader and I've chosen to endorse Ted Cruz," Walker told conservative radio host Charlie Sykes on Newsradio 620 WTMJ Tuesday.
The endorsement could be a pivotal development ahead of Wisconsin's April 5 primary, which is emerging as a key test for the remaining presidential contenders before the 2016 campaign heads east.
When Walker dropped out of the race in September, he said he hoped to clear the field.
"I believe that I am being called to lead by helping to clear the field in this race so that a positive, conservative message can rise to the top of the field," he said at a news conference last September.
An early critic of Republican front-runner Donald Trump, Walker encouraged other trailing Republican candidates to follow his path.
"I encourage other Republican presidential candidates to consider doing the same so that the voters can focus on a limited number of candidates who can offer a positive, conservative alternative to the current front-runner," he said, referencing Trump. "This is fundamentally important to the future of our party, and, more important, the future of the country."
Sykes also is no fan of Trump, having had a testy exchange with the billionaire businessman on his radio show Monday. Trump said he did not know that Sykes was part of the #NeverTrump movement before appearing on his program.
Trump did not anticipate receiving Walker's endorsement.
"After the way I beat Gov. Scott Walker (and Jeb, Rand, Marco and all others) in the Presidential Primaries, no way he would ever endorse me!" Trump tweeted Tuesday before the announcement