Say 'Happy Birthday' in Japanese: Best Messages and Wishes
“Birthday” is “tanjoubi” (たんじょうび、誕生日) or “otanjobi” (おたんじょうび、お誕生日) in Japanese. So we simply say “お誕生日おめでとう!”（Otanjoubi Omedetou) to say Happy Birthday.
When written, however, we can use a variety of different messages!! There are some phrases in Japanese that are only used in writing and not said aloud, as cited by WeXpatsGuide.
Here are different happy birthday messages in Japanese you can either say or include in their card:
To a friend
Spoken: You can also say “ハッピーバースデイ（happy basudee)” which is the Japanese pronunciation of the English phrase “happy birthday!” then we can add a simple “大好き！” (I love you!), “いつもありがとう！” (Thank you always), and “楽しんでね!”（tanoshindene) which means “Have fun!”
Written: “親友のあなたにお誕生日おめでとう。” (Shinyu no anatani, otanjoubi omedetou)” which means happy birthday to my best friend. For those you are close enough to, you can add “あなたの願いがすべて叶いますように”（Anata no negai ga subete kanaimasu youni) which means “may all your birthday wishes come true”.
To an acquaintance
Spoken: Depending on your relationship with this person, you can go with the most casual お誕生日おめでとう (otanjoubi omedetou) or add ございます (gozaimasu) at the end to make it a little more formal. This level of formality is used for people who are older than you or people you are not close enough to be very casual with.
Written: We can add “素晴らしい誕生日になりますように!”（Subarashii tanjoubi ni narimasu youni) which means wishing you a wonderful birthday or “素晴らしい一年になりますように!” (subarashii ichinen ni narimasu youni) which means “wishing you a wonderful year”.
To a superior
Spoken and written: For a superior, you must say the formal version “お誕生日おめでとうございます”（otanjobi omedetou gozaimasu) and after which we can add some words of gratitude like “いつもお世話になっております”（itsumo osewa ni natte orimasu) - “Thank you for your support” and “いつもありがとうございます”（Arigatou gozaimasu）which means “Thank you always for everything.” or “Thank you for your continued help”.
Birthday greetings addressed to between couples
Spoken: お誕生日おめでとう！(Otanjoubi omedetou) and any other words to express your love for them.
Written: “愛するあなたに お誕生日おめでとう”（Aisuru anatani, otanjoubi omedetou) which simply means “happy birthday, my love” as a term of endearment. However, this phrase is usually only written and not said aloud.
Belated birthday greetings
When you get a chance to greet someone but it’s at a later date than their actual birthday, just tell them “遅ればせながらお誕生日おめでとう”（Okurebasenagara otanjoubi omedetou) which means belated happy birthday.
For the written messages, sometimes these phrases are already pre-written in birthday cards you can buy at the store. Check the messages inside to see if they are appropriate for the person you are giving it to!
Some useful words to know for birthday in Japanese:
Birthday Cake – Ba-sude- ke-ki (バースデーケーキ)
Birthday Card – Ba-sude- ca-do (バースデーカード)
Birthday Party – Ba-sude- pa-ti-(バースデーパーティー)
Surprise Party – Sapuraizu Pa-ti- (サプライズ パーティー)
Candles – Ro-soku (ろうそく)
Balloon – Fu-sen (ふうせん、風船)
Feast – Gochi sou (ごちそう、ご馳走)