I am tax resident in Turkey and own a company abroad. Do I have to declare the dividends here?

In most cases, yes. A full tax resident in Turkey is taxed on worldwide income, and a dividend from a company abroad counts as investment income that goes on the annual return. Whether the money ever reached a Turkish bank account does not change the obligation. What matters is that the income was earned, not where it currently sits. The exception is Article 20/D: if you became resident on or after 1 January 2026, had neither a domicile nor a tax liability in Turkey in the three prior calendar years, and hold the exemption certificate, the dividend is exempt and stays off the return.