Judge Roger Chapple, at Inner London Crown Court, told Umaniec: "Your actions on the 7th of October of this year were entirely deliberate, planned and intentional."
He said it was "abundantly clear" that Umaniec was "plainly an intelligent man" and told the court he had described Rothko as a "great painter" in a letter he had written to him.
The judge also said the incident had led to galleries reviewing security arrangements at a cost to themselves and the taxpayer.
"The effects of such security reviews is to distance the public from the works of art they come to enjoy," he said.
More top news
Spencer Stone, one of the three American men who thwarted the French terror attack in August, has been stabbed repeatedly.
The 16-year-old's stepbrother and his girlfriend are accused of murdering her and then cutting her body up.
A top film producer is at the centre of an international manhunt after he disappeared, allegedly taking £45m of investment with him.