Origin: Finnish, Spanish, Japanese
Meaning: from Finnish seijas meaning "tranquil, serene, calm, clear".
Seija is a fairly modern name in Finnish, apparently inspired by a Roman goddess named Seia, who protects seeds sown into the earth; its creator, Yrjö Karilas, combined it with the Finnish word seijas and came up with Seija, which he used for his daughter in 1917. In 1924 a family friend wrote a book called The Story of Seija-girl which made the name more popular.
Seija is also a Spanish surname, a variant spelling of Ceja, which is a habitational surname. Seijas is also a variant spelling of the surname.
I also came across the name as belonging to a female character in a Japanese game meaning "right and wrong" or "good and evil" (正邪) though I have a feeling this name was created specifically for the character in this game
Seija is pronounced say-yah, the j being silent.