In August 2006, Dimitris signed a two-year deal with Panionios from Ethnikos Asteras. However, he didn't impress anyone as he scored 6 goals through these two years.
In July 2008, Sialmas signed a two-year deal with Ethnikos which lasted for only one. Then, Giannina approached him and signed him, as PAS thought that he was an extremely talented striker. Giannina fans loved him, something that intrigued the Volos club and that led Olympiakos to borrow him for 6 months.
On the 30/5/11 Sialmas signed a three-year deal with AEK Athens on a free transfer keeping him at the club until 2014. Sialmas also chose the number 11 Jersey previously worn by former AEK player Demis Nikolaidis. His skills and movements reminded ΑΕΚ fans of the legendary Demis Nikolaidis and many of them compared the two. Sialmas scored two goals against Levadiakos and helped AEK to win 0-2. He was also given the MVP award.
In the summer of 2012 Sialmas signed for Azerbaijan Premier League side Khazar Lankaran on a one-year contract. Sialmas scored six goals in 24 games in all competitions for Khazar, as well as reaching the 2012–13 Azerbaijan Cup final. In the final the game went to penalties after finishing 0-0 after extra time, Sialmas took and missed the opening Khazar penalty, resulting in Neftchi Baku winning 5-3 on penalties. Sialmas was released at the end of his contract in the summer of 2013.
On the 12/12/2013 Sialmas signed for Superleague Greece side Platanias on a 1 1/2 year contract.
In January 2015, Dimitris Sialmas decided to stay abroad, although he had offers from Greek Super League clubs to be repatriated. The Greek striker will continue his career in Ceahlăul Piatra Neamţ for the next six months, which struggling for remaining in the first division of the Romanian championship.
In March 2016, Sialmas moved to Swedish side Akropolis IF.
In August 2016, Sialmas agreed with Panachaiki, a club playing in Greek 3rd Division.
On 30 August 2017 he moved to Football League side Trikala. On 21 September 2017 he made his debut and scored his first goal, with a beautiful bicycle kick, in a 3-1 away win against Sparti for the Greek Cup. However before the final whistle he was injured and one day later the medical diagnosis confirmed that he had suffered a cruciate rupture. He is expected to return in five months.