Widnes went down 28-6 to Castleford Tigers at Leigh Sports Village on August 10, but the match was overshadowed by a subsquent pitch invasion that saw Widnes supporters appear to provoke the home fans.
Photos of the incident emerged on social media, including one showing the head of Castleford's tiger mascot being kicked away, and Widnes went on to issue an unreserved apology, condemning the actions of those involved and pledging to work with the RFL to investigate the "disgraceful scenes".
An independent tribunal has now found Widnes to be guilty of "serious breaches" of RFL operational rules, although 50 per cent of the sanction imposed on the club has been suspended until the end of the 2015 season.
A statement on the RFL's official website confirmed the punishment and added: "Widnes were found to be in breach of Operational Rule D1.8c (improper conduct), D1.10b (safety/misconduct of spectators) and the RFL's Respect Policy.
"Rule D1.10b states that clubs are 'responsible to the RFL for the actions of their spectators and all persons purporting to be their supporters, and are required to use their best endeavours to take all precautions to ensure that their spectators and all persons purporting to be their supporters do not threaten, abuse or assault third parties'.
"Clubs must also ensure that 'their spectators, or people purporting to be their supporters, do not damage property, or invade the pitch, or pitch surrounds, or throw missiles on the pitch, or act in a disorderly or improper manner before, during or after the conclusion of matches'.
"Widnes were instructed to pay £10,000 immediately with a further £10,000 suspended until the end of the 2015 season. The tribunal also ordered the club to pay £4,000 costs.
"The tribunal acknowledged Widnes Vikings' full cooperation with the RFL's investigation but felt the sanction should reflect the seriousness of the incidents."
A subsequent joint statement from the RFL and Widnes outlined both parties' acceptance of the tribunal's verdict.
RFL chief operating officer Ralph Rimmer said: "I am sure that the fine would have been significantly greater but for the response of Widnes Vikings, whose actions since the semi-final have been exemplary.
"I would like to thank the club for its full cooperation and response to what was a dark day for everyone."
Vikings chief executive James Rule added: "As a club we apologise once again for the events that unfolded at Leigh Sports Village.
"We will continue to work with the Police, the RFL and Halton Borough Council to implement banning orders to all involved.
"Whilst we accept the tribunal’s ruling and recognise our responsibilities as to the behaviour of our fans, it is important for us to recognise that the acts were not representative of our true supporters and the spirit of Widnes Vikings."