Abstract: Qualitative research with a case study design aims to describe the development of Catholic- based school culture in the Middle School (SMP) Maria Gorreti Semarang (MGS), viewed from aspects: 1) the birth process of Catholic-based school culture, 2) values the character of Catholic-based school culture, 3) the strategy of developing a Catholic-based school culture, 4) supporting factors and obstacles faced in developing a Catholic-based school culture, and 5) efforts to minimize the constraints faced in developing a Catholic-based school culture. Data collection is done by methods: in-depth interviews, participant observation, and study documentation. Determination of data sources is done by using purposive sampling technique. Data analysis was performed using the interactive patterns of Miles and Huberman (1994). Checking the validity of the data is done by using a degree of credibility through both source and method triangulation techniques. The results of study showed that he development of Catholic-based school culture in the MGS Middle School was carried out with reference to the founding father's philosophy of the Semarang Marsudirini foundation. The philosophical values of the founders of the school became the main foundation in shaping the school's cultural character. With the strategy of developing a school culture that is realized in various programs and activities aimed at teachers and education personnel and students, it is able to make MGS Middle School become an outstanding school as well as a favorite school in the city of Semarang. Keywords: Development, School Culture, SMP Maria Gorreti Semarang Abstrak: Penelitian kualitatif dengan rancangan studi kasus ini bertujuan untuk mendeskripsikan tentang pengembangan budaya sekolah berbasis agama Katolik pada Sekolah Menengah Pertama (SMP) Maria Gorreti Semarang (MGS), dilihat dari aspek: 1) Proses lahirnya budaya sekolah berbasis agama Katolik, 2) Nilai-nilai karakter budaya sekolah berbasis agama Katolik, 3) Strategi pengembangan budaya sekolah berbasis agama Katolik, 4) Faktor pendukung dan kendala yang dihadapi dalam pengembangan budaya sekolah berbasis agama Katolik, dan 5) Upaya untuk meminimalisir kendala yang dihadapi dalam pengembangan budaya sekolah berbasis agama Katolik. Pengumpulan data dilakukan dengan metode wawancara mendalam, observasi partisipan, dan studi dokumentasi. Penetapan sumber data dilakukan dengan teknik purposive sampling. Analisis data dilakukan dengan menggunakan pola interaktif Miles dan Huberman (1994) Pengecekan keabsahan data dilakukan dengan menggunakan derajat kepercayaan (credibility) melalui teknik triangulasi baik sumber maupun metode. Hasil penelitian menunjukkan bahwa pengembangan budaya sekolah berbasis agama Katolik pada SMP MGS dilaksanakan dengan mengacu pada nilai-nilai filosofis pendiri (founding father) Yayasan Marsudirini Semarang. Nilai-nilai filosofis para pendiri sekolah tersebut menjadi landasan utama dalam pembentukan karakter budaya sekolah. Dengan strategi pengembangan budaya sekolah yang direalisasikan dalam berbagai program dan kegiatan yang ditujukan bagi guru dan tenaga kependikan serta para peserta didik mampu menjadikan SMP MGS menjadi sekolah berprestasi sekaligus sekolah favorit di Kota Semarang. Kata Kunci: Pengembangan, Budaya Sekolah, SMP Maria Gorreti Semarang