It stands between Ecuador and Peru, and, even though it only spans 1km (0.6 miles), the Peruvians are not happy.
On Monday, Peru recalled its ambassador to Ecuador in protest, saying the construction violated a 1998 peace treaty.
Ecuador says it is a flood precaution.
Peru argues that disrupting the water flow could raise the flooding risk on its side.
There are also concerns it will affect informal trading between residents.
The border between the two countries runs over more than 1,500km (900 miles), but the new wall spans a tiny stretch between Huaquillas in southern Ecuador and Aguas Verdes in Peru.
It stands under four metres (13ft) high, running alongside a canal.
To cross the border between Aguas Verdes in Peru and Huaquillas in Ecuador, all you need to do is jump over a narrow channel of stagnant water.
Although most people prefer to use small, informal wooden bridges.
On both sides are chaotic markets, where both Ecuadorian dollars and Peruvian are accepted.
“For breakfast I sometimes buy bread in one country and butter in the other,” one local joked to BBC Mundo.
Yet although the new construction has prompted strong reactions from officials, this is no Donald Trump-style wall.
It takes just minutes to walk from one end to the other.
The Ecuadorean government said it “lamented” Peru’s decision to bring its ambassador back to Lima for discussions, adding that it would not allow the move to “paralyse work it is carrying out on its own territory”.
“The Government of Ecuador has built a wall… in spite of requests made by Peru to stop the work,” said the Peruvian foreign ministry.
The two countries had a one-month militarised border conflict in 1995 over disputed jungle territory. Dozens of people were killed.
The current diplomatic spat is not a good start to relations under two relatively new leaders.