Travelling across borders in Eastern Europe can be a challenge, especially between cities like Belgrade and Timisoara that aren’t connected by rail. In fact, there aren’t even buses that run between Serbia and Romania.
With Serbia not being an EU member, there are limited options to get between the capital and the underrated Romanian city of Timisoara. However, if you’re willing to get creative, it’s possible.
Here are the few options you have:
Driving From Belgrade to Timisoara

The only efficient way to get from Belgrade to Timisoara is by car. It’s a 3-hour drive along the popular European E70 highway.
Several rental car companies in both cities allow you to take cars across the border, though there will be a hefty fee to do so. Keep in mind that you’ll likely have to return the car to the location where you picked it up, which can be very inconvenient for travelers.
Taking the Train and Getting a Taxi at the Border

The only way to get between these two cities by public transport involves taking a train to the border towns and then having a taxi or driver take you across to the neighboring country’s border town.
If coming from Belgrade, you would take the train to Vrsac. Then, you’d find a driver for the border crossing to Moravita, Romania. It’s about 20 minutes between the towns. If coming from Timisoara, you’d do the same thing. However, Moravita is so small that it might be difficult to find a taxi there, so this is much more doable from Belgrade than from Timisoara.
Hiring a Private Driver

The most feasible way to get from Belgrade to Timisoara is by finding a company or private driver. There are plenty of companies to choose from in Belgrade, such as GeaTours, with minibuses taking people between the cities. You could also use a service like DayTrip to book a ride in a regular car. Serbian Insiders is a reputable tour operator based in Belgrade that can help you out as well.
In-Summary: Belgrade to Timisoara

Only the most adventurous travelers should consider getting to Timisoara by anything other than your own car or another driver. There are no trains or direct buses between the cities, so you would be stuck trying to find a taxi in one of the border towns to take you across the border.
Thankfully, Serbia and Romania are two very affordable countries to visit, so finding a local company with minibuses or even a private driver isn’t terribly expensive.
I highly encourage spending a couple of days in Timisoara. The people were so friendly, the old town was stunning, and there’s a lot of history.