KPN shares were down 3.7% to 2.80 euros by 0815 GMT as investors saw the deal reducing the chances of a lucrative takeover of KPN.
ABP will own 50% of the JV with KPN holding an option to buy one additional share and gain control of the project.
The JV will aim to add 910,000 business and home fibre optic connections by 2026, mostly in "underserved" areas, KPN said.
KPN's own plans call for adding 2.5 million households in the same period. Last year it surpassed a third of the 8 million households in the Netherlands.
In October and November 2020, with KPN's share languishing around 2.20 euros, KPN dismissed rumours it was in talks about a possible takeover by Sweden's' EQT, which also owns some fibre optic networks in Belgium and southern Netherlands.
Since then, minority shareholder America Movil held by Mexican tycoon Carlos Slim -- whose 2013 attempt to buy KPN foundered on poison pill defences -- has increased his stake in KPN to 20%, making it less likely any takeover at a decreased price would succeed.
A Dutch law declaring telecommunications networks as vital infrastructure also went into effect in October, giving The Hague a veto over any foreign purchase of KPN.
(Reporting by Toby Sterling; editing by Jason Neely)