Ealing Police Service posted a picture of the blade on Wednesday (May 17) on Twitter.
The knife measures around 16 centimetres, with the blade around seven centimetres.
Officers said in the tweet: "Brought to school by an 11 year old! Investigation ongoing.
"Do your children need advice from you too? Education=Prevention #SaferSchools."
Getwestlondon has approached the Metropolitan Police for comment.
It comes after another 11-year-old carried a knife with him into a primary school in Hillingdon .
The boy had the knife in his coat pocket the entire day on Thursday May 11, with staff only becoming aware of the blade when pupils reported it to them and was confirmed with the boy's parents.
Parents at the school in the Hillingdon borough received an email later that evening confirming that a Year 6 pupil had brought a knife with him into the school.
In a separate incident in February earlier this year, police seized a knife measuring 17 centimetres, with a blade measuring nine centimetres, from a 12-year-old pupil in the borough.
The Year 7 student said he brought the knife in to "show his friend".
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