SummaryIn plants, genetic manipulations are commonly performed using Agrobacterium tumefaciens, a plant pathogen whose abilities as a ‘natural genetic engineer’ have been co‐opted for biotechnological applications. In the widely used binary vector systems, Agrobacterium already contains a plasmid with most of the genes needed for virulence and is transformed with a second vector carrying the DNA of interest that will be transferred to the plant. Many widely used binary vectors have not been substantially improved during the past few decades, however, and often have legacy features that are not needed for many contemporary applications, such as cumbersome cloning sites that introduce sequence ‘scars’ between proteins of interest and any fusion protein, including GFP. Here, we introduce the assembly vector (AVEC) plasmids, a series of modular vector backbones that are easily customized using inexpensive DNA assembly cloning methods and that are ideal for testing multiple epitope tags, fluorophores, or non‐coding sequences, such as promoters or 5′/3′ untranslated regions. We show that pAVEC plasmids match or exceed the performance of commonly used binary vectors for transient and transgenic expression of proteins fused to fluorophores and the proximity labelling tool, TurboID. To illustrate the versatility of the pAVEC system, we demonstrate that replacing an herbicide resistance gene with a silencing suppressor gene increases protein expression in cells by an order of magnitude. We anticipate that the pAVEC system will accelerate investigations of plant molecular biology and that the modular, intentional design of these plasmids will be useful to investigate future improvements to binary plasmid design.