After returning to Liverpool Semmy found himself behind David Raven in the pecking order and despite having some good games for the reserves last season he found himself out of favour with the Reds and was not offered a new contract.
Otsemobor said: "It was always going to be wrench leaving a club like Liverpool, especially when you see what the boys have just achieved. But I need to play first-team football and so I was really pleased that the manager allowed me to talk to other clubs.
"Rotherham are an ambitious club and I could sense the disappointment with what happened last year and how determined everyone is here to have a real crack at going straight back up.
"The manager is bringing in plenty of new faces and we might take a bit of time to get to know each other on and off the pitch, but I am sure we will all be working as hard as possible from now on to ensure we get the club back up again.
"Personally, I am now really looking forward to the start of the season and to challenging for a first-team place. The buzz of running out in front of a big crowd is something I have missed and with teams like Nottingham Forest, Bradford City and Brentford in the division, there are going to be some very exciting games."