A court in the south-western Antalya province gave Rifat Cetin a suspended one-year jail sentence and stripped him of parental custody rights.
He has insisted his images, comparing Mr Erdogan with the grotesque-looking Gollum in 2014, were harmless.
Gollum appears in JRR Tolkien's trilogy as "a small, slimy creature".
His longing for the ring has distorted his body and mind.
In 2014, Cetin published on Facebook three photos of Mr Erdogan, then a prime minister, beside three pictures of Gollum with similar facial expressions.
The defendant later described the post as "harmless".
But on Thursday, the court in Antalya sentenced Cetin to a year in prison, suspended for five years.
Article 299 of the Turkish penal code states that anybody who insults Turkey's president can face a prison term of up to four years.
However, Cetin said he would appeal because Mr Erdogan was not president at the time the pictures were published, Turkish media report.
Turkey's record on freedom of speech and treatment of journalists has come in for criticism in recent years, with Mr Erdogan's administration seen as being increasingly intolerant of criticism.
Almost 2,000 cases for insulting Mr Erdogan have been opened in recent years. They include a former Miss Turkey and a 12-year-boy.
Another man, Bilgin Ciftci, is accused of insulting the president for sharing images comparing Mr Erdogan and Gollum.
Charges have also been filed against a comedian in Germany for reading an apparently insulting poem about the Turkish president.
A Turkish man has been found guilty of insulting President Recep Tayyip Erdogan for depicting him as the Gollum character from the Lord of the Rings.