In the Diamond & Pearl series, Ash appeared with another new outfit, consisting of a black vest, white undershirt, new jeans, new sneakers, and his hat's Poké Ball is blue.
Since entering the Sinnoh region Ash is less of a hot head when it came to Pokémon battles but kept it when Pokémon were being mistreated while still keeping his other traits also he is more open up to advice and being more patients to others. Ash is beginning to be less cocky due to meeting a rival who only wanted to bring power out of a Pokémon and is beginning to take some of his rival more serious much like he was with Gary while keeping his love for Pokémon. Ash is once again in a role of mentor for a another rookie trainer named Dawn much like he was for May.
Season 10: Diamond and Pearl
Departing for Sinnoh, Ash once again left his previous captures at Professor Oak's laboratory, with the exception of his Aipom who'd snuck along. After arriving in Sinnoh by boat and reuniting with Brock, he gained yet another new traveling companion in Dawn, a confident girl interested in following in her mother's footsteps as a Pokemon Coordinator. After receiving new clothes from his mother in the mail, Ash gained a new rival in a Trainer named Paul. Later in the season, Ash misses meeting Team Galactic due to moving on with his journey too soon. He won his first two Sinnoh Gym Badges: a Coal Badge from Roark and a Forest Badge from Gardenia. Ash encountered J, who temporarily kidnapped Pikachu by turning it into stone. Ash's first Sinnoh capture was a Starly which evolved into Staravia, and a Turtwig. His final Sinnoh acquisition came when he adopted Paul's Chimchar by the end of the season.
Season 11: DP Battle Dimension
In this season, Ash continued battling and earning badges while Dawn continued to earn contest ribbons. They also encountered Team Galactic and tried to stop them from stealing the Adamant Orb. During this time, Ash earns his fifth Gym Badge. For most of the season, Ash had to wait for Gym Leader Fantina to return to her Gym in order to challenge her. Ash reunited with May during the Wallace Cup and took part as a coordinator once again (per Hoenn Pokémon League Champion Wallace's request). Late one night with Pikachu, he saw the spirit of Azelf rise out of the lake the contest was being held at. He tied with Maylene but earned a Cobble Badge from her and defeated both Crasher Wake and Fantina for the Fen and Relic Badges. He also acquired a new rival in Barry, a somewhat clumsy boy who insists on fining people. Ash acquired a few more Pokemon during this season; at the suggestion of Zoey, he traded his Aipom for Dawn's Buizel, the only trade so far he hasn't reversed; his Turtwig evolved into Grotle; and he caught a Gligar which became a Gliscor.
Season 12: DP Galactic Battles
Continuing his journey in Sinnoh, Ash managed to beat Byron, winning the Mine Badge and Candice, winning the Icicle Badge. After his victory over Candice at the Snowpoint Gym, Ash briefly reunited with Brandon from the Battle Pyramid back in Kanto's Battle Frontier and witnessed a full Six on Six battle between him and his rival Paul. It was then after that battle that it was Ash's turn for his full battle with Paul which occured at Lake Acuity. However, Ash was not victorious in his battle with Paul even after Chimchar evolved into Monferno. Along the trip to Sunyshore City, Ash and his friends encounter International Policeman Looker and were recruited to help stop the plans of Team Galactic as the three of them were chosen to share a bond with the three Lake Spirit Pokémon, during which Brock managed to find Uxie. Managing to find Team Galactic in the middle of creating a new universe with the captive Lake Trio controlling Palkia and Dialga, Ash, Dawn and Brock freed the trio and Team Galactic was arrested while their leader vanished into the collapsing new world, effectively disbanding Team Galactic. At the end of the season, Ash gives Gliscor to the Air Battle Master named McCann for some training and has caught himself a Gible which was learning Draco Meteor.
Season 13: DP Sinnoh League Victors
After the Team Galactic incident, Ash entered the Sinnoh League after beating Volkner and earning the Beacon Badge. He also reunited with Jasmine from the Olivine Gym before heading to Lily Of the Valley. Taking a different approach in this League than he did in Hoenn, Ash combined his previous captures with his current Pokemon to form well-balanced teams of new and old. The Sinnoh League was one of the hardest and most intense Leagues he ever competed in, as he had rivals Paul and Barry to compete with, and a mysterious trainer who joined with the Legendary PokémonDarkrai. He battled against most of his rivals save for Barry, who battled Paul and lost, so Ash's next match was against Paul.
The battle was intense as Paul showed Pokémon that he didn't normally use while Ash had reassembled his team of six from their earlier battle at Lake Acuity. The fight came down to Paul's Electivire and Ash's Infernape. Despite taking heavy damage, Infernape won the match for Ash with its ability Blaze. After defeating Paul, Ash faced an opponent other Trainers in the League dreaded to face as he was hard to defeat: the Trainer with the Darkrai, Tobias. After losing three of his Pokemon to it, Ash was able to defeat Tobias' Darkrai with Sceptile and even knocked out Tobias' Latios with Pikachu. However, as the rest of Ash's Pokémon had fainted, he lost the match and was eliminated. To this day, the Sinnoh League remains Ash's most impressive performance in a Pokémon League Tournament: he reached the Top 4 - the farthest he'd ever gotten in any Pokémon League - and was the only one of Tobias' opponents to defeat two of his Pokémon, let alone his Darkrai. After the Sinnoh League, Ash bade farewell to Dawn as she wished to continue entering contests, and also parted ways with Brock, his oldest friend and longest-running traveling companion for the final time, as Brock now wished to become a Pokémon Doctor.