Sieg made his NASCAR debut in 2009 in the Camping World Truck Series, entering the Kroger 250 at Martinsville Speedway in a No. 21 Dodge for GunBroker Racing. He retired with an ignition problem after starting sixteenth. He subsequently ran a partial schedule at Milwaukee Mile for his family-owned team, recording six top-twenty finishes in nine races, including a ninth at Gateway International Raceway.
In 2010, Sieg competed across 21 of 25 Truck Series races with no major sponsorship, earning two top-ten finishes — an eighth at Kentucky Speedway and a ninth at Dover International Speedway. His best Truck Series result came on June 10, 2011, when he finished seventh in the WinStar World Casino 400K at Texas Motor Speedway in his 44th career series start, after leading ten laps following a pit strategy call.
Sieg joined the Xfinity Series full-time in 2014 under the RSS Racing umbrella, finishing ninth in the season-opening DRIVE4COPD 300 at Daytona for his first top-ten in the series. At the July Daytona race, he drafted Kasey Kahne to victory and secured a third-place finish for himself, his first career top-five, which earned him an entry into the Dash 4 Cash competition at New Hampshire. He finished sixteenth in the 2014 standings.
In 2015, Sieg ran with Uncle Bob's Self Storage sponsorship and finished eleventh in points, with a highlight eighth-place result at Kansas. For 2016, he qualified for the inaugural Xfinity Series Chase, though he was eliminated in the first round. June 2017 brought his career-best Xfinity finish — a second place at Iowa Speedway. That Darlington throwback weekend, Sieg's car honored his brother Shane, who had died two weekends prior.
His 2018 season saw RSS Racing expand to two start-and-park entries to help fund Sieg's No. 39 effort, and he qualified for the Xfinity Dash 4 Cash at Dover after Spencer Gallagher's eligibility was stripped. A pit road penalty ended his Dash 4 Cash run early. In 2019, Sieg qualified for the Xfinity Series playoffs, though he was later disqualified from the Las Vegas race for minimum ride height violations — a disqualification that did not affect his championship standings.
The 2020 season marked a high point for Sieg and RSS Racing, when he recorded a career-best seven top-five finishes and advanced to the playoffs' Round of 8, ultimately finishing tenth in points. RSS Racing switched from Chevrolet to Ford for 2021.
At the spring 2022 Las Vegas race, Sieg reacted to light contact from Ty Gibbs by attempting to retaliate — instead spinning and collecting Sheldon Creed and Brett Moffitt. The incident drew scrutiny. Despite the controversy, Sieg advanced to the 2022 Xfinity Series Playoffs later that fall following a tenth-place finish and two additional penalties against competitors at Bristol, before being eliminated in the Round of 12.
At the 2024 Texas race, Sieg finished second to Sam Mayer by 0.002 seconds, one of the closest finishes in series history. Later that season at Talladega, Sieg claimed the Dash 4 Cash bonus for the first time in his career. For the final five races of 2024, Ryan and Kyle Sieg swapped car numbers — Ryan moving to the No. 28 to accumulate owner's points, Kyle taking the No. 39.
In the 2025 Cup Series replacement at Phoenix, Haas Factory Team tapped Sieg to substitute for suspended driver Sam Mayer before the Championship race.
RSS Racing has served as both Ryan Sieg's car and the team he operates. The organization has grown from a single underfunded Truck Series effort to a multi-car Xfinity Series program, at various times fielding start-and-park entries alongside Sieg's primary No. 39 to supplement funding. The team switched manufacturer affiliations from Chevrolet to Ford in 2021. Kyle Sieg joined the team as a full-time driver, making RSS Racing a family operation across multiple entries.
Sieg has made sporadic Cup Series starts. His debut came in May 2017 at Dover, driving BK Racing's No. 83 Toyota Camry. A month later he returned for BK at Michigan, substituting in the No. 23. In 2019, Sieg ran the Brickyard 400 at Indianapolis Motor Speedway for Premium Motorsports in the No. 27.
Ryan Sieg represents the independent racer archetype in modern NASCAR — a driver who has funded and operated his own program while remaining competitive at the Xfinity level over more than a decade. Without the backing of a major manufacturer alliance or a fully resourced ownership group, RSS Racing has repeatedly qualified for the Xfinity playoffs and achieved top-five finishes, demonstrating what disciplined budget management and experienced driving can accomplish in the sport's second tier.