Police in the canton of Solothurn said on Twitter the fire was brought under control at 1pm. They said windy conditions had made battling the blaze difficult and that the bridge would remain closed until further notice.
Footage from earlier in the day showed flames and smoke billowing from the 200-year-old covered bridge.
Police said earlier there was no information on injuries at this stage. The extent of any damage caused was also unknown.
The bridge, which dates back to 1803, is one of the landmarks features of Olten, a commercial and transport hub in striking distance of Basel, Bern and Zurich.