On his podcast, Actions Detrimental, Hamlin openly admitted to walling Chastain on purpose during the final restart of last weekend's race won by Hendrick Motorsports driver Alex Bowman.
The two drivers have a feud dating back to last June, clashing in multiple races since then.
Hamlin said on Monday: "It wasn't a mistake. I let the wheel go and I said he's coming with me."
NASCAR has decided to take action, docking Hamlin 25 driver points and fining him $50,000.
The sanctioning body cited two sections of its Members Code of Conduct in assessing the penalties. Section 4.4B prohibits attempts to manipulate the outcome of a race or championship - wrecking or spinning another vehicle, whether or not that vehicle is removed from Competition as a result, while 4.4D covers actions by a NASCAR Member that NASCAR finds to be detrimental to stock car racing.
Elton Sawyer, NASCAR's senior vice president of competition, said the sanctioning body likely would have viewed the end-of-race contact between Hamlin and Chastain "as a racing incident."
But he said Hamlin's public admission on the podcast meant NASCAR was forced to intervene.
"When you start admitting that you have intentionally done something that would compromise the results of the end of the race, then that rises to a level that we're going to get involved," Sawyer said.
"There's no other way to look at that. We were going to get involved in that situation.
"We've been consistent in the past with that and we will be consistent going forward."
The two drivers have had several run-ins dating back to Gateway last summer. They clashed several times after that, notably at Atlanta Motor Speedway and then Pocono Raceway. In fact, it was Hamlin that Chastain eliminated from the playoffs when he famously rode the wall into the Championship 4 at Martinsville.
The 2023 season began with yet another incident in the pre-season exhibition race in Los Angeles, with Chastain spinning Hamlin.
Meanwhile, NASCAR also hit all four of Hendrick's teams and Justin Haley's Kaulig Racing team with its biggest penalties yet for Cup Series teams it found had altered parts from a single-source supplier on the Next Gen car.
Hendrick's Kyle Larson, William Byron and current points leader Bowman, plus Kaulig driver Haley, have all been docked 100 driver points, 100 owner points and 10 playoff points.
Hendrick's #9 car that was driven last weekend by Josh Berry, in the absence of Chase Elliott through injury, was deducted 100 owner points and 10 playoff points but no driver points.
In addition, all five crew chiefs of the respective teams were fined $100,000 and suspended four races each.
The violation was noted as from Sections 6.1.A Time/Manner/Location; 14.1 C&D&Q Overall Assembled Vehicle Rules; 14.5.4.2.A Radiator Duct Note: Unapproved modification of a single source vendor supplied part (hood louvers).
Hood louvers are vents in the hood on each manufacturer’s car that serve as a release point for ducts that transfer air out of the radiator. The use of vents prevents the need of teams’ to tape up the front grille of car, which had been used extensively as a performance adjustment tool in the previous iteration of race car.
During initial inspection last Friday at Phoenix Raceway, NASCAR inspectors discovered issues with the hood louvers from all four HMS cars, which were confiscated after practice. NASCAR also confiscated the hood louvers from Kaulig’s #31 Chevrolet prior to Saturday’s qualifying session.
The teams were allowed to replace them and take part in time trials, while the illegal louvers were taken back to NASCAR’s Concord research and development centre for further evaluation.
Hendrick plans to appeal the penalties, releasing the following statement: "On Friday at Phoenix Raceway, NASCAR identified louvers on our race cars during voluntary inspection 35 minutes after the opening of the garage and prior to on-track activity.
"NASCAR took possession of the parts approximately four hours later with no prior communication. The situation had no bearing on Saturday's qualifying session or Sunday's race.
"We are disappointed with today's decision by NASCAR to issue penalties and have elected to appeal based on a variety of facts."
However, it has made the 'strategic decision' to not ask for a deferral of the crew chief suspensions.
With the introduction of the Next Gen car in the 2022 season, NASCAR updated its penalty system to include much harsher consequences for violations, including the revoking of playoff eligibility, in particular with respect to modifying parts from single source suppliers.
Front Row Motorsports, Roush Fenway Racing and Stewart-Haas Racing all were hit with severe penalties last season for altering single source supplied parts.
