The # symbol was first used to label channels on Internet Relay Chat in 1988. IRC was created by Finnish programmer Jarkko Oikarinen at the OuluBox bulletin-board system, and its channel naming convention helped turn the symbol into an enduring marker for online groups and topics.
On IRC, a channel is a shared conversation space. A name such as #chat or #news identified a channel available across an IRC network, while other prefixes could indicate different channel scopes. The symbol was therefore being used as a channel prefix long before Twitter popularized hashtags for searchable subjects.
The date is sometimes described as “around 1988” because IRC evolved through early implementations rather than appearing as one finished system on a single launch day. The protocol was later documented more formally in RFC 1459, published in 1993. That later date explains a common mix-up: 1993 marks an important standardization milestone, not the original use of # for IRC channels.