This survey explores artifacts like steles, metal or stone statues, metal foils, and coins, bearing inscriptions in the Sanskrit language and Siddhamātṛkā (or “Siddham”), Nāgarī, and Proto-Bengali/Gauḍī scripts produced in Java between the 8th and 13th century CE, contextualizing them against the background of the pan-Asian networks of Tantric Buddhism or Mahāyāna Buddhist Tantra and especially its circulation along the maritime “Silk Routes.” Discussing the interrelationship between languages, scripts, religions, and politics in Java and relevant regions of the wider Buddhist world, it tries to answer questions concerning foreign or local agency and audience as well as transregional connectivity. In particular, it argues that the quick spread of varieties of Mahāyāna/Mantrayāna Buddhism from the Subcontinent to Java and East Asia during a “first wave” from the 8th to the 9th century appears to have occurred in parallel with the diffusion of Siddhamātṛkā script in those locales, whereas a “second wave” of Tantric Buddhism linking the Indo-Tibetan and East Asian Buddhist world is associated with Nāgarī and Proto-Bengali/Gauḍī script in East Java.