The Democratic presidential candidate said he needed “a lot more” evidence on whether the substance caused further drug use.
The US National Institute on Drug Abuse (NIDA) says most cannabis users do not go on to use harder substances.
"There has not been nearly enough evidence that has been acquired as to whether or not it is a gateway drug,” Mr Biden said during a town hall meeting in Las Vegas.
“It’s a debate. I want a lot more before I legalise it nationally. I want to make sure we know a lot more about the science behind it.”
One study found that even though cannabis did often precede people taking other drugs, the reason why subjects used cannabis in the first place - such as "boredom" - was a larger risk factor than the cannabis itself.
Mr Biden said he supported individual states deciding their own recreational cannabis laws.
He said: “I don’t know enough to know whether it is [a gateway drug] or not, although I’ve done a great deal of work on the drugs side of the issue.”
The former vice president under Barack Obama was asked during the meeting whether his stance towards recreational cannabis has changed. He replied: “No, it has not.”
Mr Biden has a long history of supporting anti-drug legislation and has been criticised for his role during the "war on drugs" of the 1990s.
During that period, he helped write a “tough on crime” bill which led to harsh sentences for crack cocaine use and has been accused as disproportionately affecting black Americans.
However, during his talk in Las Vegas, he said he did not believe people should be sent to prison for cannabis use and that any existing convictions should be expunged.
He also said he supported medical use of the drug.
One study has claimed cannabis may be the opposite of a gateway drug, as it could prevent people from becoming hooked on potentially dangerous prescription drugs.