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Wagner (31) agreed to terms with the county champion and will join it next month.
His deal is through to the end of July, with an option to stay on for the remainder of the season, subject to New Zealand Cricket approval.
Essex head coach Anthony McGrath hopes Wagner can wield the same influnce as 2017.
‘‘We have been working hard on bringing Neil back for a while,’’ he said.
‘‘His performances were excellentlast season. He showed great commitment to the club last year and was integral to what we achieved.’’
Wagner has been a consistently successful performer for the Black Caps at test level and is rated the world’s seventh-best bowler in that format.
Essex has also recently signed Australian pace bowler Peter Siddle for the first five games of the 2018 Championship season.