What kind of U.S. visa do I need to visit my current employer's office in the U.S.?

Everything the other posters have said seems accurate. Three more things:

You said that you would be working in the US office. As the others have pointed out, that’s generally not a problem with a B-1, as long as the work is consistent with a business visit (which it usually would be), and as long as you are paid by the German office according to your German salary.

If you are going to be paid a US salary, things get more complicated. For a one-week trip, that’s very unlikely, so don’t worry about it. But just in case: in that case, you would need an L-1 visa. That is something your employer would have to get for you; you can’t do it on your own. It also involves a lot more paperwork, and money.

The second comment: you shouldn’t have to do this all by yourself. If a company is large enough to be multinational, they should have an immigration attorney available who can answer these questions specific to your particular situation.

The third note: since you are a Belarussian citizen, the US Consulates in Germany may refuse to even process your application. You may have to travel to Belarus to apply for your visa. I’m not sure about their exact policies.

Other answers say that you need a B-1/B-2 visa, but this is not precisely true. As a business visitor, you will be in B-1 status as a "visitor for business." B-2 designates the closely related status of a "visitor for pleasure," which covers tourism and visits to family and friends, for medical treatment, and for most other purposes that are not related to business.

To be admitted in B-1 status as a business visitor, you need a B-1 visa. The State Department usually, but not always, issues the visa as a B-1/B-2 combination visa, which allows you to be admitted in either B-1 or B-2 status.

It seems that the State Department is more likely not to issue a combination visa when the reciprocity terms for business and pleasure visitors are different. You can see at https://travel.state.gov/content/travel/en/us-visas/Visa-Reciprocity-and-Civil-Documents-by-Country/Belarus.html that they are not different for nationals of Belarus, so you will probably get a B-1/B-2 visa, but you might get a B-1 visa. Either is fine for this trip.

Visitor visas for citizens of Belarus are valid for no more than one year, so if you know that you will make a separate non-business trip to the US within one year of your application, you can ask specifically for a combination visa. Otherwise, it doesn’t matter whether you get a B-1/B-2 combination visa or a simple B-1 visa.

Note that "Fee: none" in the table linked above means that there is no additional "reciprocity" fee on top of the normal visa application fee, currently USD 160 for most nonimmigrant visas, including B visas.

You’d need a B1/B2 visa. You can apply for it via your nearest US consulate. You can apply it online first, and schedule a physical interview on the nearest available date. You’ll need to explain your purpose of visit and perhaps also provide some documents proving your intent. Like an invitation letter from your employer stating your location of visit, purpose and tenure of stay. Your visa on approval will be pasted on your passport and is usually valid for 10 years. However on entry to the US, you will be interviewed again by an officer at the airport, who will decide how long you can stay in the US. Your last day of stay will be mentioned in what is called the i94 document which can be accessed online. You are not supposed to work/take monetary benefits during this visit as it is purely a business visit.

All the best!

You will not be working. You will be there on business. This is exactly what a B1 (usually issued as a combined B1/B2) is for. You will be visiting a business and having meetings there. (This answer to a similar question gives a nice list of things you can do on business.)

"Working", in customs-speak, involves not just doing things but doing things and getting paid for doing them by an American company. Business visits like yours are entirely different from say, waiting tables at a restaurant, roofing or other construction work, or delivering lectures.

A company large enough to have offices in multiple countries should have someone who helps their staff get visas. Talk to whoever is sending you on the trip to get a little help and advice with it.