President Biden's Transportation Secretary made the announcement Saturday, writing ... "Chasten and I are beyond thankful for all the kind wishes since first sharing the news that we’re becoming parents. We are delighted to welcome Penelope Rose and Joseph August Buttigieg to our family." He attached a photo of him and Chasten holding their kids.

The siblings are wrapped in blankets, while Chasten and Pete look at each other in a hospital bed. Presumably, these newborns are adopted -- it's no secret the couple have been trying to undertake the process for a good while ... and now, they're at the finish line.

Looks like their children might be twins ... but it's not entirely clear if they adopted from one birth parent or two. The babies are also about a month old now -- Pete first broke the news of their arrival back in mid-August, and this is the first time he's shared a pic.

He wrote at the time, "For some time, Chasten and I have wanted to grow our family. We're overjoyed to share that we've become parents! The process isn't done yet and we're thankful for the love, support, and respect for our privacy that has been offered to us."