Matthew Scully-Hicks, 31, assaulted and abused 18-month-old Elsie over seven months before he violently shook her to death.
The jury of nine men and three women have been retired since Wednesday afternoon but came back with a unanimous verdict.
The child died just two weeks after she was formally adopted by fitness instructor Scully-Hicks and his husband Craig, 36.
Scully-Hicks denied murdering little Elsie but was found guilty by a jury after a four week trial.
Cardiff Crown Court heard the youngster suffered a string of injuries when she was alone with stay-at-home dad Scully-Hicks.
Speaking after the trial Senior investigating officer T/Detective Chief Inspector Stuart Wales of South Wales Police Major Crime Investigation Team said: "Our thoughts today are with little Elsie and those who knew and loved her.
"Her untimely death at just 18 months old has had a devastating effect first and foremost on her family, who remain upper-most our thoughts.
"Elsie’s death has also impacted a wider community, including the many professionals involved in her care and the subsequent investigation. I would like to thank all of them, including the many witnesses who assisted the prosecution.
"Matthew Scully-Hicks has now been found guilty of murder.
"This case represents an extremely rare and distressing set of circumstances. We at South Wales Police continue to respect and value the role that adoption, and those involved, play in our society."
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