Siak may refer to:

• Asiagh (असियाग) (Siak (सियाक)), a gotra/clan of Jats found in India
• Siunia Dynasty, alternatively known as Siak, a dynasty that ruled part of what is now Armenia
• Eric Low Siak Meng (born 1948), Singaporean Chinese politician
• Siak Regency, a regency of Riau, Sumatra, Indonesia
• Siak River, a river in Sumatra, Indonesia
• Sultanate of Siak Sri Indrapura a kingdom in Siak Regency from 1723 to 1946
• PS Siak, a football club based in Siak Regency

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