The pair arrive at Ewood Park following centre-half Shane Duffy's departure to Brighton for an undisclosed fee.
Williams, 23, made 131 appearances for the Robins, helping the club to the League One title in 2014-15.
Hoban, 22, has played 58 games for Watford, scoring twice.
Both players, who have Republic of Ireland Under-21 honours, will be available for Saturday's Championship game against Fulham.
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Blackburn Rovers have signed defender Derrick Williams for an undisclosed fee on a three-year deal from Bristol City, while Tommie Hoban has joined on a season-long loan from Watford.