The 14 poems were found in a book titled "Black Island Album," named after the house in central Chile which Neruda, his third and last wife Matilde Urrutia and her niece Alicia Urrutia shared, according to Nurieldin Hermosilla.
The lawyer and Neruda collector said he bought the book recently from a book dealer, who in turn had acquired it from an anonymous seller.
The poems are handwritten in Neruda's traditional green ink and are "a direct and definitive confirmation from the poet's own pen of his love for Alicia," Hermosilla said.
He said Alicia Urrutia decided to go public with the poems after years of keeping silent about her affair.
"I think she decided to confirm her love with Neruda and put this book on sale to lend herself some legitimacy and put an end to the myth" and speculation, the lawyer said.
Neruda, who died at 69 in 1973, just 12 days after dictator Augusto Pinochet's coup, is famous for his love poems as well as his "Canto General" -- an epic poem about South America's history and its people.
But he also was a senior member of the Chilean central committee of the Communist Party and his work was banned during Pinochet's 1973-1990 military dictatorship.
At different periods of his life, Neruda was a political exile, a senator, an ambassador and in 1970 a presidential candidate for the Communist Party.
Neruda's real name was Neftali Ricardo Reyes Basoalto. His pen name was inspired by Czech writer Jan Neruda.