Jackman joined the Cobblers from Gillingham last summer and agreed a two-year deal at Sixfields.
He has now been rewarded with an additional 12 months to tie him to the League One club until 2010.
The 25-year-old made 44 appearances for Northampton in all competitions last season and was voted the club's Player of the Year.
Cobblers boss Stuart Gray told the club's official website: "Danny Jackman has agreed to this contract extension and that is great news.
"Danny deserves his extension beacuse he is a very good player who enjoyed a good season last season.
"He has quality and a first-class attitude. He is a sought-after player and he is adaptable which is very important."