Jason Kelce, a mellow mountain of a man, has spent most of his life fretting over offensive snaps.

Now, a year into his retirement after 13 seasons as a center for the Philadelphia Eagles, Mr. Kelce has space to puzzle over other matters. Lately, he’s been thinking about sweatshirts and T-shirts. And he has thoughts. Lots of thoughts.

“On the inside of a sweatshirt, I don’t like when it feels fuzzy,” said Mr. Kelce, speaking on a video call from his Philadelphia home in April, just a few days after the birth of his fourth daughter. He wants a “heavier-duty” sweatshirt, with a smidge of stretch. Something durable, “that you can wear working or lounging around on the couch.”

Oh, and it needs to be made entirely in the United States.

To achieve all this, Mr. Kelce, 37, has taken matters into his own hands. Today, his Underdog apparel brand is debuting a tidy collection of crew neck sweatshirts and T-shirts, all produced — from the cotton to the completed garment — in the United States. These items are produced in collaboration with American Giant, a San Francisco-based company formed in 2012 that manufactures entirely in the United States.

The American Giant-produced Underdog pocket T-shirts and French terry sweatshirts will sell for $45 and $79, respectively, and will be sold on the Underdog website. They are, to Mr. Kelce’s specifications, on the burlier side. During the interview, he wore the pocket T-shirt in green, his abundant biceps mildly squeezed by the sleeves.

“I don’t want to offend anybody, I never understood the reason that somebody would want to wear Gucci, or some of these high-end brands,” said Mr. Kelce, leaving unsaid that his brother, Travis, has been known to wear a Gucci hat or two. “It’s never appealed to me.”

“I would much rather wear something that symbolized a blue-collar worker as opposed to a suit,” he added, even if he does have to wriggle into a suit when he appears as an analyst for “Monday Night Countdown” on ESPN.

That Underdog’s clothes are entirely made in the United States is the most gratifying part to Mr. Kelce.

“I grew up going into steel mills with my father,” he said of his childhood in Cleveland Heights, Ohio. “So, the concept of American workers making things was something that was ingrained early. And I just always gravitated toward it.”

Mr. Kelce has long been plotting for life after his last snap. In 2022, he started the “New Heights” podcast with his brother. It remains in the top 200 of Apple’s top podcasts, though it now runs behind his wife Kylie’s show, “Not Gonna Lie.”

While many retired athletes pour money into car dealerships or restaurant ventures, Mr. Kelce became curious about clothes.

“I was trying to come up with what’s a way that we can celebrate Philadelphia,” said Mr. Kelce, who is nothing if not an unofficial ambassador for his adopted city. “And I love wearing shirts that represent me and everything that I believe in.”

He started Underdog in 2022 as an apparel brand with vigorous nods to Philadelphia. Past graphics featured his Eagles teammates, and the name alludes to the Eagles having labeled themselves as underdogs on their way to winning the Super Bowl in 2018. It is a unique proposition from a former athlete, and one that has, to many outside the Eagles fandom, perhaps flown under the radar. Underdog does not have the techy edge of say, Tom Brady’s slick, fitness-centric Brady brand. It leans more on Mr. Kelce’s jolly, jumbo-size profile than on his on-field prowess.

The label has made prior attempts at producing in the United States, though Mr. Kelce soon learned that even if the tag said an item had been made here, “a good portion of that might be made overseas.” Garments that read “Made in the United States” can be produced from cotton or other components that derive from elsewhere.

In American Giant, Underdog found a partner that was capable of producing the whole garment, including the fabric, here in the United States. The cotton was harvested and finished in the Southeast, including in North Carolina. The garments themselves were produced in Los Angeles.

“These fabrics were custom-designed for Jason,” said Bayard Winthrop, the chief executive of American Giant who underscored that the ex-Pro-Bowler “had lots of small opinions” about how the shirts and sweats should fit and feel.

“The shirts have a really gutsy, sturdy quality to them,” said Mr. Winthrop. “They drape a certain way. They’re not clingy to the body. They’re not overly delicate.”

Last year, Mr. Kelce traveled with Mr. Winthrop to North and South Carolina see the production process firsthand, giving him a look at how clothes are made that few people ever see, or even consider. With most apparel manufacturing having been exported overseas, only 2 percent of the clothing Americans buy is manufactured domestically.

“The eye-opening part for me was how truly decentralized this is, how many people touch just one T-shirt and how many steps there are in that process,” said Mr. Kelce. “I had never really thought much about that.”

Though this project has been in the works for around two years, it debuts at an auspicious moment, as Americans are contemplating what they’re willing to sacrifice — macro- and micro-economically — to bring back American manufacturing. Hours before the interview, President Trump pulled back on steep tariffs that had unsettled the global economic landscape, though a tit-for-tat trade war with China has continued.

“I have no issues with global trade whatsoever,” said Mr. Kelce, who was evenhanded about the impact of his small stab at making his goods solely in the United States. He was, he said, not going to stop buying shirts and clothes that were not totally made here.

“I don’t think there’s any type of statement trying to be made other than, ‘Listen, there’s some really awesome reasons to support homegrown American businesses and local businesses,’” he said.

Considering Mr. Kelce’s longstanding love affair with Philadelphia, it may strike some shoppers as odd that the Underdog line is not produced there. It was discussed, but the reality, said Mr. Winthrop, was that Philadelphia, like many cities that were once manufacturing hubs, was “gutted out” when it came to apparel production.

“There is no at-scale dyeing or finishing or knitting capability in Philly,” he said. Mr. Kelce said that in the future, the company would find ways to incorporate the city into its brand. His partner, Stephen Porter, noted that anything Underdog screen printed or embroidered was done locally.

The line might not be made in Philadelphia, but his green shirt proved that Mr. Kelce knew he had to keep his fans happy.

“Philadelphia bleeds green,” he said. “If we didn’t have a green shirt, it would have felt like malpractice.”