Short answer in Jain Dharma:

• For monks and nuns: Any sexual activity, including masturbation, is strictly prohibited and is a serious transgression of the vow of brahmacharya.
• For laypeople: Masturbation is considered a breach of the brahmacharya-anuvrata (limited vow of chastity). It excites passions (kashāyas), strengthens attachment and delusion, and causes karmic bondage (pāpa). Therefore, it is discouraged and counted as a fault.

Why it is a fault (simple Jain view):

• It stirs rāga–dveṣa and moha, opening the flow of new karmas (āśrava) and bondage (bandha).
• Brahmacharya exists to calm the senses and conserve inner energy for dharma; self-indulgence goes against that purpose.
• The intensity of pāpa depends on intention, imagination, frequency, and negligence (pramāda).

If a lapse happens:

• Alochanā: Acknowledge it honestly.
• Pratikraman: Do repentance the same day, with clear remorse.
• Pratyākhyān (pachchakhan): Renew a vow to avoid it for a set time.
• Guard the mind-body-speech (guptis), keep to samayik, svādhyāy, satsang, sleep on time, avoid triggers, and follow simple, saatvik food discipline.

Digambar and Śvetāmbara notes:

• Both traditions uphold absolute celibacy for ascetics and treat masturbation as a transgression of brahmacharya for laypeople. The practical path is vigilance, repentance, and renewed restraint.

Further reading:

• Layperson’s Twelve Vows, including Brahmacharya Anuvrat
• Guidance on sexual conduct in Jainism
• Q&A: Masturbation in Jain view