Morgan said on Twitter that he has decided to "try pastures new" after the station's boss asked him to remain for another two years.
The deal would have seen him host a series of 40 interview specials.
Morgan took over from veteran talk show host Larry King, but his show was axed in March due to falling ratings.
The primetime programme launched in January 2011 to audiences of around two million, but they dwindled to some 270,000.
He said: "I had a fantastic time in my four years at CNN and have huge respect for Jeff [Zuckerberg, CNN boss] and all the people who work there.
"Great company, great network," added the former Britain's Got Talent judge.
In December 2012, thousands of Americans lobbied for his deportation from the country after he advocated gun controls in the wake of the Connecticut school shootings.
He continued to be an ardent opponent of the gun lobby while on air and said as he left CNN: "I haven't finished with you lot yet."
Shortly after Morgan made his debut on CNN, his predecessor King said that he had been "oversold" to
US TV audiences.
"He's certainly not bad. He's certainly an acceptable host. He asks good questions, maybe he interrupts a little too much at times. I think he may have been oversold," said the veteran broadcaster.
Morgan has yet to give any indication of his next career move, also writing on his Twitter feed:
"Ironically, my transfer window just opened."
He also confirmed his contract had come to an end on Tuesday but added: "@piersmorgan left @CNN a
long time ago."
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