Roger filed the defamation lawsuit Thursday against Geoff Payne for saying Roger was supposed to take care of Liam at the time of his death. According to the docs, obtained by TMZ, Roger says Geoff submitted a sworn declaration to Buenos Aires police stating Roger had "full responsibility for Liam's care during this last period when I was not there."

But, Roger sternly denies that saying in the lawsuit, he "never agreed to be and was never the caretaker of Liam." He acknowledges they were "dear friends" and supported each other, but says it was never a paid job for him.

Remember, Buenos Aires prosecutors have charged Roger in Liam's death ... accusing him of abandoning Liam just prior to his death.

In the lawsuit, Roger calls out Geoff for failing to share several emails between himself and Roger -- in one dated August 2024, Roger says he told Geoff he "could not further as a friend devote the same time to support Liam" and that Geoff needed to take action regarding Liam's condition ... a reference to the One Direction singer's battles with addiction.

Roger says he and Geoff actually spoke in September 2024, and he urged Liam's dad to come to Wellington, FL -- where Liam lived -- and take care of his son. He says, to the best of his knowledge, Geoff never came.

In his lawsuit, Roger points out Geoff's allegedly false statements spread around the world when TMZ obtained police documents containing his claims about Roger. Those documents were featured in our documentary "Liam Payne: Who's to Blame?" ... in which we also interviewed Roger.

According to the suit, Roger stands by something he told us -- that he wasn't lacing Liam with narcotics. He says he "never provided Liam with any recreational drugs and never controlled any prescribed medications taken by Liam."

As for who actually was supposed to care for Liam ... well, Roger's pointing fingers at a couple of people. According to the docs, he says Geoff trusted one of Liam's bodyguards, Bledar Vata, to be a "paid caretaker" -- but, when Liam fired Vata, "Geoff as self declared caretaker of Liam never provided for a temporary or new bodyguard caretaker."

He also points out Liam was living with his gf Kate Cassidy in Florida, and at a couple hotels in Argentina, in the months of September and October 2024. While he doesn't lay direct blame on her, Roger says she had "a unique capacity, opportunity and superior knowledge of Liam's activities and needs."

In the suit, Roger sums up Liam's tragic demise, saying his friend was fully independent, but struggled for years with addiction -- going in and out of treatment facilities.

Roger tells TMZ ... "Geoff needs to backtrack on his sworn statement as he very well knows I wasn't Liam's nurse or care taker. Liam, who I miss every day, was my dear friend and an independent, brilliant, respectful free man who did whatever he wanted whenever he wanted."