最澄入唐將天臺宗傳進日本，以比叡山為中心創立日本天臺宗。日本天臺宗的一個顯著特點是將天臺宗與密宗相結合。這一特色到其弟子圓仁及其後的圓珍、安然時有較大發展。 圓仁在公元 838 年作為短期的「請益僧」入唐，因故未能到天臺山巡禮求法，在回國時遭遇巨風滯留中國，展轉遊歷五臺山、長安等地，從當地名師受學天臺宗、密宗教法，其間碰上唐武宗「會昌滅佛」，一度還俗。 圓仁於公元 847 年回國，將自己經歷寫為《入唐求法巡禮行記》。在教義方面，撰寫了《金剛頂經疏》、《蘇悉地經略疏》等，提出其密教勝於天臺圓教的判教論，認為以《法華經》為基本經典依據的天臺宗不屬於顯教，而屬於密教，但其地位比以《大日經》、《金剛頂經》等為代表的密教低，因為《法華經》雖也講俗諦、勝義諦相即不二的「理密」，但沒有講大日如來宣示的身、口、意「三密」--所謂「事密」，而《大日經》、《金剛頂經》卻有這方面的內容。此外，他還依據人人具有佛性的理論，對密教一向主張的即身成佛的理論作了新的發揮，說普通的「凡夫」只要認真修持「三密」，就有可能在現身迅速成佛。圓仁還把在五臺山學到的法照之「五會念佛」的淨土法門傳回日本，為以後以比叡山為源頭的淨土信仰的興起產生很大的影響。

　　　　　After his studies in China, Saicho established the Japanese Tendai School with its headquarters on Mt. Hiei. A special feature of this school, the incorporation of tantric elements, was further developed by Saicho's disciple En-nin and later on by En-chin and An-nen. In 838 CE, En-nin went to China for a stint as a "monk requesting for instruction" but certain circumstances prevented him from the pilgrimage to Mt. Tientai, yet when he tried to return to Japan strong winds forced him to stay in China. He thereupon travelled to Mt. Wutai, Chang'an and other places and studied with famous masters the teachings of both the Tientai and Tantric Schools. During his stay, he went through Emperor Wuzong's persecution of Buddhism which forced him to return to lay life for some time. After his return to Japan in 847 CE, he recorded his experiences inthe Nittoguhojunreigyoki. In doctrinal respect, he composed among other works the Kongochokyosho and Soshitsujikyoryakusho arguing for the supremacy of the tantric teaching over the Tendai Perfect Teaching. In his view, the Tendai School which based itself primarily on the Lotus Sutra did not belong to the exoteric schools but to the esoteric. Nevertheless it was on a lower level than the esoteric school which mainly used texts like the Dari jing and Jin'gangding jing because the Lotus Sutra, despite talking about the "noumenal secret" of relative and absolute truths being identical and inseparable, did not mention the "phenomenal secret" of the "three secrets" of body, speech, and mind which were expounded by Buddha Vairocana and are mentioned in the Dari jing and Jin'gangding jing. Moreover, based on the concept of Buddha nature inherent in every human being, En-nin further developed the idea of "attaining Buddhahood in this very body", one of the basic teachings of the esoteric schools. According to him, any ordinary worldling was in a position to quickly achieve Buddhahood in this very life provided he practiced the "three secrets" seriously. En-nin also introduced Fazhao's wuhui nianfo to Japan. This pure land practice which he had learnt on Mt. Wutai exerted great impact on the rise of pure land devotionalism at Mt. Hiei.