In June 2021, Kleindienst returned to 1. FC Heidenheim by signing a contract until 2025.[7] On the final matchday of the 2022–23 season, Kleindienst scored in the 99th minute in the stoppage time to secure a 3–2 away win against Jahn Regensburg, in which his club finished top of the table and promoted to the Bundesliga for the first time in their history.[8] In that season, he also managed to achieve the top scorer of the division with 25 goals.[9]
On 6 April 2024, he scored a brace in a 3–2 victory over defending champion Bayern Munich, to be the latter's first defeat against a newly promoted side since 2000.[10] He finished the 2023–24 season with the most aerial duels won in Bundesliga in 187 occasions,[11] and also recorded the highest number of sprints, totaling 1,056.[12]
On 3 October 2024, he was called up to the senior national team for the first time.[15] Eight days later, he made his senior debut by starting in a 2–1 away victory against Bosnia and Herzegovina in the UEFA Nations League.[16] A month later, on 16 November, he scored his first international goals by netting a brace in a 7–0 victory over Bosnia and Herzegovina in the same competition.[17]