Can foreigners become police officers?

I always wondered if i moved to Japan would i be able to go into law enforcement. This is if i gain Japanese citizenship of course. Anyone?

First, you need to be a Japanese citizen to be eligible for the job, as you seem to know already. Then, you will have to pass the exam administered by each prefecture (or for an elite national police bureau job, the exam would be administered by the central government). In order to take the exam, you'd also need to be fluent in Japanese (at least high school graduate) and must be under 30 years old. Given these conditions, I would not say it is impossible, but it is nearly impossible for a foreign person to become a police officer in Japan, since one would need 5 to 10 years to become a naturalized citizen, to begin with. By that time, you'd be most likely too old to take the exam.

thats an interesting question i dont know about the rules regarding foreigners joining the Japanese Police (i think the answer will be no). However, you'll need a degree to get to Japan in the first place which means you will be 22+ then ive heard it takes 10+ years to get citzenship so maybe you will be too old by then?

You would first have to become a Japanese citizen, which can take many years (10+ if you don't marry a Japanese citizen, 5+ if you do).

You would also need to have native level speaking and writing ability, which is quite an undertaking in itself- for writing in particular you need to put in several years dedicated study to reach that level, just being in Japan doesn't get you there.

By that time as above, age may be an issue, as most new recruits are recent graduates.

Not to mention that knowing a language is not enough for a policeman, social worker etc. you need to know the thousands of things that come naturally to natives but are never explained in a school
to take an example from my culture: adults Frenchmen (and other Europeans) hug and kiss male relatives and friends. In French speaking Canada adult males don't kiss their dad, brothers etc.. They shake hands.
Had I shook my dad's hand in front of his buddies, they all would have thought I was crazy.

If the Op could become a policeman in Japan, he would be surprised daily by many things that are normal in Japan but likely not in his country. This would be a big problem.
I know that a French policeman would have a hard time becoming a policeman in Quebec, even after he/she has obtained the Canadian citizenship, because of the differences between the 2 schooling systems, and the somewhat different French spoken in the 2 countries.

The vast majority of countries require you to be a citizen/national of that country before you can become a Police Officer or other public office holder,
This is the same in the UK, Canada, USA, and Japan is no different.

While knowing the culture is important, and by the time a foreigner does become a national of Japan they would I assume know a bit more then the average foreigner would.

However understanding 100% of it doesn't make someone a good or not good police officer. To be a good police officer means you have a open mind that not all people are the same, there are many police officer in the USA that come from different backrounds, cultures, languages who become American citizens and are effective police officers.