The court based in Saint Gallen agreed with the country's competition commission (Comco) that the former state-owned company acted in an uncompetitive way in the high-speed internet domain from 2005 to 2007.
But the court reduced the penalty imposed by Comco of 220 million francs in a judgment made public on Tuesday that could be further appealed to the supreme court.
Before the complete liberalization of the Swiss telecommunications sector in 2007, other companies in the field were reliant on Swisscom as a provider for high-speed Internet access.
But Swisscom charged rates that made it impossible to offer the service without losing money, the court's judges determined.
They concluded that Swisscom had violated the federal law on cartels.