Watkins has been banned for four years after he tested positive for the anabolic steroid nandrolone and the stimulant methylhexaneamine.
He joins team-mate Shaun Cleary, who has been banned for two years as benzoylecgonine, a cocaine metabolite, was found in his system.
Both players tested positive before a friendly against Bridgend Ravens RFC.
UKAD Director of Legal, Graham Arthur said: "Ryan Watkins deliberately ingested nandrolone and methylhexaneamine without any consideration for his responsibilities as an athlete.
"By making this conscious choice to dope, Watkins has chosen to cheat his team-mates, the opposition and his sport.
"I hope this case will act as strong deterrent to other young amateur players - the risks to your playing career, your reputation and more importantly to your health, just aren't worth it."
Speaking about Cleary, Mr Arthur said:  "Although Mr Cleary used cocaine three days before playing, cocaine was still in his system when he played.
"Cocaine is banned from sport and athletes are solely responsible for what is in their system, regardless of whether there is an intention to cheat or not."
Welsh Rugby Union chief executive Martyn Phillips said about Cleary: "This case serves as a strong warning to everyone in the game that non-compliance with anti-doping rules carries grave consequences.
"Whether intentional, or inadvertent, players have a responsibility to themselves, to each other, to their clubs and to the sport to act within the rules and spirit of the game.
"We work closely with UK Anti-Doping and fully adhere to the World Anti-Doping Code. There is no room in the code for carelessness or not knowing.
"We will be relentless in working with UKAD to follow up leads that out players who dope in Welsh rugby."
Maesteg Harlequins are mid-table in Welsh National League Division One West Central and Cleary has been banned until 10 October, 2017. Watkins is banned until 11 September 2019.

The Care Quality Commission (CQC) made a number of recommendations including ensuring nursing staffing levels were safe at Sandwell General Hospital and City Hospital in Birmingham.
It also called for improvements in A&E, medical care and surgery.
Sandwell and West Birmingham Hospitals NHS Trust said it is "working hard already to address the issues".
Care at the trust was given an overall rating of "requires improvement".
The CQC said: "The trust was rated as 'inadequate' with regard to whether [it] was always providing a service that is safe."
Source: CQC inspection between 14-17 and 25-30 October
The trust said the report made for  "difficult reading".
Chief executive Toby Lewis said: "The areas of improvement identified in the report are not a surprise to us at the trust, even where they are disappointing, or make difficult reading."
CQC's chief inspector, Prof Sir Mike Richards, said: "While we witnessed areas of good practice it is clear that the trust has work to do to make improvements which are sustainable long term."
Inspectors will return to check whether the necessary changes have taken place and the CQC will continue to monitor the trust "closely".
Mr Lewis said: "During the CQC visit, inspectors saw some individual examples of poor practice in some departments around hand washing, security of medicines and completion of patient records."
He said these individual examples "are not acceptable" and it is up to the trust leadership to ensure it gets "the basics right, first time, every time for every patient".
Maesteg Harlequins lock Ryan Watkins has become the 12th Welsh rugby player to be suspended by UK Anti-Doping.
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The safety of services at a trust running two hospitals has been rated "inadequate" by a health watchdog.