In a speech in Istanbul, Moore also said Putin has "bitten off more than he can chew" and underestimated Ukrainians in the war.
"He seeks to impose his imperial will by all means at his disposal. But he cannot succeed," he said.
"Bluntly, Putin has bitten off more than he can chew. He thought he was going to win an easy victory. But he - and many others - underestimated the Ukrainians."
Moore was speaking at the British consulate in Istanbul after five years as head of MI6. He leaves the post at the end of September.
During his tenure, Russia launched an invasion of Ukraine in February 2022, a war that has seen tens of thousands killed and still rages, principally in eastern Ukraine.
Moore said the invasion had strengthened Ukrainian national identity and accelerated its westward trajectory, as well as pushing Sweden and Finland to join NATO.
"Putin has sought to convince the world that Russian victory is inevitable. But he lies. He lies to the world. He lies to his people. Perhaps he even lies to himself," he told a news conference.
He said Putin was "mortgaging his country's future for his own personal legacy and a distorted version of history" and the war was "accelerating this decline."
Moore, who previously served as the UK's ambassador to Ankara, the Turkish capital, added that "greater powers than Russia have failed to subjugate weaker powers than Ukraine."
Analysts say Putin believes he can outlast the political commitment of Ukraine's Western partners and win a protracted war of attrition by wearing down Ukraine's smaller army with sheer weight of numbers.
Ukraine, meanwhile, is racing to expand its defence co-operation with other countries and secure billions of dollars of investment in its domestic weapons industry.
with AP
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