The N-Dubz singer is charged with helping an undercover journalist obtain 13.9g of cocaine for £860.
She was due to stand trial alongside co-defendant Mike GLC, a rapper from North London, who has pleaded guilty.
Tulias and Mike, real name Michael Coombs, sat side-by-side in the dock and Tulisa spoke clearly to state her name and plead not guilty.
Michael then pleaded guilty and kissed Tulisa on the cheek before leaving the dock.
Judge Alistair McCreath told the rapper, who previously denied the charge, that he will be sentenced at the end of Tulisa’s trial.
"Michael Coombs, I'll sentence you at the conclusion of the proceedings against your co-defendant, whatever that conclusion may be,” he said. "In the meantime I'll permit you to keep your bail, but you must understand that that consists of no sort of indication as to what sentence you'll get at the end of it.
"On that understanding you may go."
Tulisa attended a preliminary hearing in April, following her arrest in July 2013 and an initial hearing in December 2013.
The police investigation followed a police sting, in which Tulisa was allegedly filmed appearing to help with a cocaine deal for a man who was later revealed to be an undercover journalist. Tulisa was arrested after the incident along with musician Mike GLC.
In December, the singer's lawyer Ben Rose gave a statement, explaining that his client would plead not guilty. According to The Mirror, he said: "Tulisa has been charged with a serious criminal offence to which she will plead not guilty.
"As has been widely reported, this entire case has been manufactured by the Sun on Sunday and Mazher Mahmood, sometimes known as the fake sheikh.
"They spent a large amount of their readers' money in flying Tulisa and a number of her friends first class to Las Vegas."
A spokesman for The Sun has said: "The Sun on Sunday's investigation into Tulisa Contostavlos was entirely justified in the public interest. It was undertaken by Mazher Mahmood, our award-winning journalist.
"Throughout our investigation, our team followed the Press Complaints Commission Code and then handed over our dossier of evidence to the police.
"Following the police investigation, prosecutors have decided that there is a clear case to answer. It is right that this matter should go to court and be decided by a jury."
Tulisa and her assistant Gareth Varey, who has been accused of threatening behaviour, are both on unconditional bail.