Eboney Cheshire suddenly fell ill at her home in Rainhill, St Helens, Merseyside, on the night of December 2.
Police and paramedics were called and Eboney was rushed to hospital at 11.40pm but tragically died the next day at 1.20am.
The teen’s cause of death was initially described by Merseyside Police as "unexplained”.
But the post-mortem has now confirmed Eboney’s cause of death was MDMA, the chemical name for ecstasy.
A Merseyside Police spokesman said today: "We can confirm a Home Office post-mortem has confirmed the cause of death of Eboney Cheshire, 13, from Rainhill, who died on 3 December.
"The post-mortem determined the cause of death was Methylenedioxymethamphetamine (MDMA) Toxicity."
Tributes flowed for the popular Rainhill High School pupil following her tragic death.
Her devastated mum Kerry Williams described her daughter as a “beautiful princess”.
She said: "My beautiful baby girl, I love you with all my heart.
"I am broken and can't think of a life without you. I don't want to see tomorrow, I just want you back in my arms. You are my best friend, so funny, full of character and so popular. I'm broken without you and our lives will never, ever be the same.
"My baby girl, I love you more than life itself and your brother is heartbroken. Fly, my beautiful princess.
"Your mummy is heartbroken. I hope you realise how much I love you and how I would have swapped places with you in a heartbeat. I begged them baby girl, but they wouldn't let me."
Eboney’s heartbroken brother Leon Cheshire said he will never forget the moment he found out his "beautiful sister" was dead.
He said: "The hardest thing in the world is losing someone you love.
"I promise whoever is reading this and I've told people the same thing, no matter what the relationship is between brother, sister etc show them that you love them.
"Spend as much time with them as possible, if they pay no interest make sure they understand you love them, just as much as they love you.
"Because people do care and I wish I [had] told my sister I loved her more than I ever did.
"I love [you] my little angel".
Eboney was laid to rest at St Bartholemew's Church in Rainhill on January 2.
Police said the investigation into Eboney’s death remains ongoing.
