si is a share identifier. YouTube started attaching it to links produced by the Share button in 2023, and it connects the person who clicks to the share that produced the link. The video plays identically without it.
If you noticed that YouTube links suddenly got longer and uglier a couple of years ago and never found out why, this is the answer.
What a shared YouTube link looks like now
https://youtu.be/dQw4w9WgXcQ?si=hK3nR9pQ2mF7xW
Before si arrived, the Share button produced https://youtu.be/dQw4w9WgXcQ and nothing else. The video ID is the string after the slash. That is all a browser needs.
The si value is generated per share. Two people sharing the same video produce two different links to the same content, which is the tell that it describes the share rather than the video.
Why this one bothers people more than most
Ordinary tracking parameters attach to advertising clicks, which most people accept as the cost of a free web even if they do not enjoy it. si is different in a way that I think explains the reaction to it.
You get it by pressing Share. You are doing the platform a favour, sending it traffic, and the link it hands back has a tag on it. Then you paste that into a family group chat and the tag propagates to everyone who taps it.
I built the YouTube rules for this site not long after si started appearing, and it was the parameter that generated by far the most email. Not because it does anything dramatic, but because people felt slightly cheated by it. A share link should be a link to the thing.
The other YouTube parameters
si is not alone. Depending on where you copy from, a YouTube URL may carry:
t is the start timestamp in seconds. This one is functional. If you set a start time before copying, t is what carries it, and cutting it means the video restarts from zero.
list and index carry playlist context. Share a link with these attached and the recipient inherits whatever playlist you were in rather than just the video you meant to send.
pp is an encoded parameter carrying playback and surface context.
feature records where the click came from, such as share or an embed.
ab_channel appends the channel name and shows up when copying from certain views.
The distinction between t and the rest is the whole problem with cleaning YouTube links by hand. Cut at the ? and you lose your timestamp along with the tracking, which is annoying enough that most people give up and share the messy link.
Redirect links in video descriptions
A related mechanism worth knowing. Links in a video description are not direct. YouTube wraps them:
https://www.youtube.com/redirect?q=https%3A%2F%2Fexample.com&v=dQw4w9WgXcQ
The real destination sits URL-encoded inside q. The click is counted on the way through. If you have ever copied a link from a description and wondered why it was so strange, that is why.
How to clean a YouTube link
By hand, keep the video ID and the timestamp, drop everything else:
https://www.youtube.com/watch?v=dQw4w9WgXcQ&t=42&si=hK3nR9pQ&list=PLxyz&ab_channel=Someone
https://youtu.be/dQw4w9WgXcQ?t=42
youtu.be is YouTube's own short domain, so the second form is not a third-party shortener. It is the canonical short version of the same address.
The youtube url cleaner does this conversion and keeps the timestamp, which is the part people most often lose doing it manually.
Related reading
Instagram does the same thing with igshid, described in what is igshid.
Spotify uses a parameter also called si for the same purpose, covered in how to clean Spotify links.
Everything else: the URL tracking parameter glossary.
Frequently asked questions
Does removing si break the video?
No. The video ID is the only required part.
Will I lose my timestamp?
Only if you cut the whole query string. Keep t and the start time survives.
Is youtu.be safe?
Yes. It is YouTube's own domain, not a third-party shortener.
Why do two people sharing the same video get different links?
Because si identifies the share, not the video.
Does si let YouTube see who I sent the video to?
It connects clicks back to a share event. What YouTube infers from that depends on what else it knows about the people clicking. It is not reading your chat.