What was the first hashtag to become a trending topic on Twitter?

The first hashtag to become a trending topic on Twitter was #sandiegofire. It gained attention during the October 2007 San Diego County wildfires, when residents used it to collect fast-moving updates about fires, road closures, evacuations, and safety information.

Chris Messina had proposed using the hash symbol to group Twitter messages only weeks earlier, in an August 2007 tweet about #barcamp. Twitter did not initially adopt the idea as an official feature, but the San Diego fires demonstrated its practical value. Nate Ritter, a San Diego resident, posted frequent updates and used #sandiegofire so others could follow the emergency in one stream.

A common mix-up is confusing the first hashtag ever proposed with the first hashtag to gain broad public use. #barcamp was Messina’s original example, while #sandiegofire was the early breakthrough that showed hashtags could organize breaking news. Twitter later added clickable hashtags and formal trending-topic features, helping turn the user-created convention into a central part of online communication.