The weapon in question was a kettlebell, which is used for weight training.
Jail records show Diddy - whose real name is Sean Combs - was released on bail on Monday evening.
The bail posted was $160,000 (£101,000).
Officials did not identify the victim of the assault or say what led to the incident.
In a statement, campus police said no-one was seriously injured.
'Unfortunate incident'
Combs' son Justin Combs had been working out on the campus. He has played just a handful of games for the UCLA football team since joining three years ago.
Snoop Dogg's son Cordell Broadus also plays on the same team.
Football coach Jim Mora thanked his staff for their professionalism in handling the situation.
"This is an unfortunate incident for all parties involved,'' Mora said in the statement from campus police.
"While UCPD continues to review this matter, we will let the legal process run its course and refrain from further comment at this time."
Combs' representatives did not immediately respond to requests for comment.
It is not the first time Combs has found himself in trouble with the authorities over his conduct.
In 1999, he was arrested for his involvement in the beating of former Interscope executive Steve Stoute in New York.
Combs apologised, the charges were reduced, and he was ordered to attend an anger management class.
The rap mogul topped a Forbes list of the five wealthiest hip-hop artists last year.
Diddy led with an estimated fortune of $700 million (£416m), an increase of $120 million (£71m) over his net worth in 2013.
That increase was largely down to his new, music-focused, multi-platform channel - Revolt TV - of which he is the majority owner.
It was the second year running the rapper topped the rich list.
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