And the club are negotiating with veteran Australian paceman Andy Bichel about a return to Essex for the second half of the summer.
Despite the short-term nature of Nel's involvement, which is still subject to final approval by Cricket South Africa, Essex chief executive David East said he was delighted by the signing.
"Andre was a huge asset to us in the brief time he was at Essex in 2005. He is a wholehearted competitor and is sure to make a significant early season impact in both one-day and Championship cricket," he told the club's website.
Nel took wickets with his first and third balls on his Championship debut for the club and was bowling at the death when they sealed their Totesport League title against Hampshire Hawks at Colchester.
He is currently preparing for a Test series against India and looks set to add to his 23 caps, which have brought him 84 wickets at a cost of 29 runs each.