It means she faces spending the next three decades in a standard Spanish prison cell, as the jury rejected an alternative verdict which would have allowed her to serve her sentence in a psychiatric wing.
Members agreed Smith, from Lichfield, Staffordshire, did suffer from an unspecified psychosis and from depression - but ruled she was not criminally insane.
They said she 'took advantage of the trust the children gave her as their mother' and their 'defencelessness' as sleeping children.
Judge Adolfo Garcia Morales will sentence her at a later date.
Outside court, prosecutor Victor Pillado Quintas said: "Today justice has been done for two defenceless small children who were murdered in the most abominable and vile way - by their own mother."
After the verdicts, the judge and lawyers in the case joined members of the jury in a local bar. During a four-day trial, the court heard how Smith killed the children after her fugitive partner, TV hypnotist Martin Smith, was arrested on child sex charges at their home in Barcelona.
She became obsessed with the idea that UK social services would put the youngsters in care - which she considered a fate "worse than death". She asked her daughter if she would prefer to spend a few brilliant days with her mother or a horrible lifetime without her.
When Rebecca said she wanted to stay with her mother, Smith took the children to the resort of Lloret de Mar to give them "a perfect holiday".
They checked into room 101 of the Hotel Miramar on May 15, 2010.
After two days of playing together and posing for seaside snaps, she put them to bed at about 7pm and waited an hour until they were asleep. Then she took a plastic shopping bag and suffocated them.
The next day - having tried to commit suicide - she instructed reception to summon Spanish police.
During her trial Smith refused to answer more than 100 questions from prosecutor Victor Pillado Quintas. The jury were shown a chilling video of Smith calmly confessing to detectives as she sat on a bed in the next-door hotel room. She told officers: "I have ended the lives of my two children. I gave the children a three-day holiday, a perfect holiday. I knew they were going to take my children to England.
"My intention was that my children and me together would go. It wasn't just the children."
The court heard Smith was "pathologically obsessed" with her partner Martin Smith - they shared their surname but she was not his wife. As she was led away to the cells last night, British social services faced questions about their role in the tragedy.
Both Cumbria and Staffordshire social services knew Rebecca was potentially at risk when Martin Smith was arrested on November 1, 2007, on suspicion of raping his step-daughter Sarah, Smith's elder daughter from a previous relationship.
Yet they allowed the family to flee to Spain before Rebecca could be taken to safety.
Last night a blame game erupted as Cumbria social services said they had passed the file to Staffordshire, who said they were relying on the police to chase the family in Spain. Meanwhile the Spanish equivalent of social services claimed: "Nobody put us on alert and so nobody took any measures to protect the children."
Even after Martin Smith was deported to the UK, Staffordshire social services did not ask its counterpart to retrieve Rebecca, and two weeks later she was dead.
A Staffordshire council spokesman said: "During the short time the family were resident in Staffordshire, the council did everything possible to initiate safeguarding procedures, and have assisted police fully in their continued investigations into this case."
- DAILY MAIL
