Karate Baka Isekai

After losing a fight with a runaway truck, a karateka is transmigrated to another world, one full of swords and sorcery straight out of fantasy books. The goddess who sent him there offers him a superpowered cheat skill to assist him in his upcoming heroics—but naturally, he turns her down. After all, he already possesses the only technique he needs: karate! No matter what manner of otherworldly foe he may face, be they a minotaur, a gigantic steel-clad knight, or a wicked sorcerer, none of them will discourage him from walking his own path in the way of the empty hand!

(Source: J-Novel Club)