Wilkinson told Sunderland's official website: "I've been aware of Mart since he first came into English football with Portsmouth and I have followed his progress at Derby very closely.
"Mart is a top-quality goalkeeper with lots of experience and I know he's very ambitious to improve and succeed. I'm delighted he has agreed to join us. Mart will add to our goalkeeping strength.
"Our current injury situation means that we were running the risk of having an experienced goalkeeper like Jurgen Macho only having youngsters to support him. Going into Premiership games with untried inexperienced goalkeepers is too great a risk."
Wilkinson has been given special permission to sign Poom outside of the transfer window because Thomas Sorensen is out until the New Year with a dislocated elbow and Thomas Myhre with a thigh strain, leaving the manager with no real cover.
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