Former West Wing star Lowe joined the NBC sitcom as a series regular this year, following a two-episode guest appearance in the second season.
"Who knew Rob Lowe was funny?" Pratt said to Movieline. "It's really surprising and it's great. He's got a lot of fans and people really love him. I mean... the guy has been around for three decades, and he's just a major star."
The actor, who plays Andy Dwyer on the show, praised Lowe for "not just [playing the] straight man".
"This is a version of him that people are not used to seeing," he claimed. "He's playing a very peculiar and weird character. I think people are going to really like that."
Pratt also described working with his new co-star as "surreal".
"It's a little surreal to be on set with Rob Lowe and he's pushing me around in a shopping cart," he laughed. "I'm like, 'You've got to be f**king kidding me? I'm at work right now'."
Parks and Recreation airs on Thursdays at 9.30/8.30c on NBC.