Born in 1955 in Hiroshima, he studied composition with Isang Yun in Berlin, and later in 1983–86 with Klaus Huber in Freiburg am Breisgau. In 1980, he participated for the first time in the Darmstadt Summer Courses for New Music, where he would become a regular guest as a tutor from 1990. Till 1998 he was the artistic director of the annual Akiyoshidai International Contemporary Music Seminar and Festival in Yamagushi. Since 2001, he has held the same position at the Japanese Takefu International Music Festival in Fukui. Since 2004 he has worked as a permanent guest professor at the Tokyo College of Music. He lives in Nagano, Japan and Mainz, Germany. 

Hosokawa considers the compositional process to be instinctively associated with the concepts of Zen Buddhism and its symbolic interpretation of nature. In the instrumental work In die Tiefe der Zeit (1994), for instance, the cello represents the male and the accordion the female principle, whereas the surrounding cosmos is reflected by the strings. The orchestral work Circulating Ocean, composed as a commission for the Salzburg Festival, was premiered under Valery Gergiev. Woven Dreams, first performed by the Cleveland Symphony Orchestra at the Lucerne Festival in 2010, won a BASCA British Composer Award in 2013. 

The excellent interpreters of Hokosawa’s works include Sir Simon Rattle, Kazushi Ono, Robin Ticciati, and Kent Nagano. Several of his works take as their subject dramatic events in Japan, including the atomic destruction of Hiroshima, the 2011 tsunami and the subsequent nuclear disaster. In response to these terrible events, Hosokawa wrote, among others, the opera Stilles Meer. Meditation was commissioned by the Hamburg State Opera and saw its premiere conducted by Kent Nagano in January 2016. 

Many of Hosokawa’s works have become staples of the contemporary repertoire and earned him prestigious accolades, such as the 1st Prize in the composition competition for the centenary of the Berliner Philharmoniker (1982), Kyoto Music Prize (1988), and Japan Prize of the University of California, Berkeley (2023). In 2001 Hosokawa became a member of the Akademie der Künste in Berlin. He has been a composer-in-residence at several international festivals, including Warsaw Autumn (in 2005 and 2007). 

Selected works (since 2010): Danses imaginaires II for large orchestra (2010), Für Walter − Arc-Song II for soprano saxophone, piano and optional percussion (2010), Woven Dreams for orchestra (2010), Spell for violin (2010), Horn Concerto Moment of Blossoming (2010), Matsukaze, opera in one act (2010), Sternlose Nacht − Requiem für Jahreszeiten for soprano, mezzo-soprano, two narrators, mixed chorus and orchestra (2010–12), Blossoming II for chamber orchestra (2011), Autumn Wind for shakuhachi and orchestra (2011), Singing Garden in Venice for baroque orchestra (2011), Threnody to the Victims of Tohoku Earthquake 3.11 for viola (2011), The Raven, monodrama for mezzo-soprano and 12 players (2011−12), Meditation to the Victims of Tsunami 3.11 for orchestra (2012), Mai, old Japanese dance music for piano (2012), Etude II Point and Line for piano (2012), Nacht Klange for cimbalom (2012), MI-KO for three accordions (2012−13), Concerto for Trumpet and Orchestra Im Nebel (2013), Klage for soprano and orchestra (2013), Distant Voices for string quartet (2013), Ancient Voices − In memory of Wolfgang Schulz for wind quintet (2013), Trio for violin, cello and piano (2013–14), Lost Love Song – Tsurenaki-Hito for voice, bass recorder and string quartet (2014), Drei Engel-Lieder for soprano and harp (2014), Aeolus − Re-Turning III for harp and chamber orchestra (2014), Small River in a Distance for string quartet (2014), Fluss − Ich wollt’, ich wär ein Fluss und Du das Meer for string quartet and orchestra (2014), Hika for violin and string orchestra (2015), Nach dem Sturm for two sopranos and orchestra (2015), Stilles Meer, opera (2015), Sorrow River for recorder and strings (2016), Umarnung Licht und Schatten for organ and orchestra (2016), Sublimation for cello and orchestra (2017), Futari Shizuka, opera (2017), Erdbeben. Träume, opera (2017–18), The Flood for ensemble (2020), Violin Concerto Genesis (2020), Fanfare for Grafenegg for brass ensemble and percussion (2021).