The 28-year-old was stopped at 8.30am on Sunday as he passed through Heathrow Airport on his way home to Rio de Janeiro.
Mr Greenwald has written a number of articles about mass surveillance programmes by the US National Security Agency (NSA) since interviewing Mr Snowden.
The Guardian reported that Mr Miranda was questioned under schedule 7 of the Terrorism Act 2000 which applies only at airports, ports and border areas, allowing officers to stop, search, question and detain individuals.
"To detain my partner for a full nine hours while denying him a lawyer, and then seize large amounts of his possessions, is clearly intended to send a message of intimidation to those of us who have been reporting on the NSA and GCHQ.
"The actions of the UK pose a serious threat to journalists everywhere.
"But the last thing it will do is intimidate or deter us in any way from doing our job as journalists.