Substitute Gareth Bale scored twice - including a brilliant overhead kick - as Madrid inflicted a 3-1 defeat on Liverpool at the Olimpiyskiy Stadium in Kiev.
But Jurgen Klopp's side responded swiftly, with Senegalese striker Saido Mane equalising from close range 10 minutes into the second half.
Leading goalscorer Mohamed Salah was guided off the pitch by the Reds training staff, after suffering an apparent shoulder injury at the half hour mark.
Following his sensational goal, Bale would tack on another thanks to another error from Karius, thus sealing Real's third straight Champions League title and fourth in five years.
Madrid are the first team to achieve that feat since Bayern Munich in 1976.
Cristiano Ronaldo has now won five in his great career, but even he could not argue that this was Bale's evening.
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"If they win the final it will be an unprecedented event in their history".
Salah was replaced by Adam Lallana after 30 minutes, and the Reds went on to lose 3-1.
It was too much for Liverpool to overcome and Real confidently saw out the final minutes to begin a celebration that has become an annual occurrence in the Spanish capital.
"I had a five/six-week injury at the start of the season and I've been fit ever since. They deserve it. I'm really proud".
After collecting a shot, Karius rolled the ball out too casually, neglecting the presence of Benzema who struck out his right foot and dispatched the ball into an empty net. Madrid opponents Marcelo and Dani Carvajal tendered to Salah as an anxious Klopp held his hand to his mouth.
But their renewed hope was crushed upon the introduction of Bale.
Such a strike deserved to be the victor, although it was his long-range shot that beat the embarrassed Karius in the 83rd minute that made sure of the victory.