Telenor, which is 54 percent owned by the Norwegian state, is accused of impeding the entry of a third competing mobile network into the country between 2010 and 2014.
"Infringements of the Competition Act's ban on abuse of a dominant position are very serious, and the notified fine of 906 million kroner (€100 million, $106 million) underlines this," the competition authority's director general Lars Sørgard said in a statement.
Norway is one of the few European countries to have only two telecoms networks in Telenor and Sweden's Telia.
The authority's director Gjermund Nese said the entry of a third network is crucial to increasing competition and "guaranteeing a good supply of (mobile) services at the lowest price possible".
Network Norway and Tele2 wanted to create a third network but needed use of Telenor's network during the rollout phase to do so.
Telenor is accused of making access to its network too costly, thus damaging the third network's profitability, and of entering into exclusive agreements with four mobile operators, reducing the new competitor's chances of attracting customers.
Telenor has until March 1st, 2017 to register any objections.