As a result, Twitter’s shares dropped more than 5% on Thursday, but showed a modest rise on Friday.
In its explanation, S&P said the rating could be raised if the social network increases its revenue through international expansion and new product launches.
It gave Twitter a stable outlook because it says it expects it to experience “very strong growth”, and points out that there are no obvious likely competitors on the horizon.
The rating came just a day after Twitter announced new features including the ability to upload and share videos directly to Twitter, and customising the way tweets are shown.
Twitter’s share price is down some 40% since 1 January this year.
A 7% fall in timeline views per user – a key measure of engagement – was reported by Twitter in October.
It has also said that fourth-quarter revenue may fall short of market expectations of £286.1m.