Siddha (Sanskrit: सिद्ध siddha; "perfected one") is a term that is used widely in Indian religions and culture. It means "one who is accomplished." It refers to perfected masters who have achieved a high degree of perfection of the intellect as well as liberation or enlightenment. In Jainism, the term is used to refer to the liberated souls. Siddha may also refer to one who has attained a siddhi, paranormal capabilities.

Siddhas may broadly refer to siddhars, naths, ascetics, sadhus, or yogis because they all practice sādhanā.

In Jainism, the term siddha refers to liberated souls who have destroyed all karmas and obtained moksha. They are free from the transmigratory cycle of birth and death (saṃsāra) and are above Arihantas (omniscient beings). Siddhas do not have a body; they are soul in its purest form. They reside in the Siddhashila, which is situated at the top of the Universe. They are formless and have no passions and therefore are free from all temptations. They do not have any karmas and they do not collect new karmas.

The Śvetāmbara Ācārāṅga Sūtra 1.197 describes siddhas in this way:

The liberated soul is not long nor small nor round nor triangular nor quadrangular nor circular; it is not black nor blue nor red nor green nor white; neither of good nor bad smell; not bitter nor pungent nor astringent nor sweet; neither rough nor soft; neither heavy nor light; neither cold nor hot; neither harsh nor smooth; it is without body, without resurrection, without contact (of matter), it is not feminine nor masculine nor neuter. The siddha perceives and knows all, yet is beyond comparison. Its essence is without form; there is no condition of the unconditioned. It is not sound, not colour, not smell, not taste, not touch or anything of that kind. Thus I say.

According to Jains, siddhas have eight specific characteristics or qualities. The ancient Tamil Jain Classic 'Choodamani Nigandu' describes the eight characteristics in a poem, which is given below.

கடையிலா ஞானத்தோடு காட்சி வீரியமே இன்ப
மிடையுறு நாமமின்மை விதித்த கோத்திரங்களின்மை
அடைவிலா ஆயுஇன்மை அந்தராயங்கள் இன்மை
உடையவன் யாவன் மற்று இவ்வுலகினுக்கு இறைவனாமே

The soul that has infinite knowledge (Ananta jnāna, கடையிலா ஞானம்), infinite vision or wisdom (Ananta darshana, கடையிலா காட்சி), infinite power (Ananta labdhi, கடையிலா வீரியம்), infinite bliss (Ananta sukha, கடையிலா இன்பம்), without name (Akshaya sthiti, நாமமின்மை), without association to any caste (Being vitāraga, கோத்திரமின்மை), infinite life span (Being arupa, ஆயுள் இன்மை) and without any change (Aguruladhutaa, அழியா இயல்பு) is God.