Registered memory (also called buffered memory) is computer memory that has a register between the DRAM modules and the system's memory controller. A registered memory module places less electrical load on a memory controller than an unregistered one because the register locally re-drives the signal. Registered memory allows a computer system to remain stable with more memory modules than it would have otherwise.

When conventional memory is compared with registered memory, conventional memory is usually referred to as unbuffered memory or unregistered memory. When registered memory is manufactured as a dual in-line memory module (DIMM), it is called an RDIMM. Similarly, an unregistered DIMM is called a UDIMM or simply "DIMM".

Registered memory is often more expensive because of the additional circuitry required and lower number of units sold, so it is usually found only in applications where the need for scalability and robustness outweighs the need for a low price – for example, registered memory is usually used in servers.

Although most registered memory modules also feature error-correcting code memory (ECC), it is also possible for registered memory modules to not be error-correcting or vice versa. Unregistered ECC memory is supported and used in workstation or entry-level server motherboards that do not support very large amounts of memory.

Normally, there is a performance penalty for using registered memory. Each read or write is buffered for one cycle between the memory bus and the DRAM, so the registered RAM can be thought of as running one clock cycle behind the equivalent unregistered DRAM. With SDRAM, this only applies to the first cycle of a burst.

However, this performance penalty is not universal. There are many other factors involved in memory access speed. For example, the Intel Westmere 5600 series of processors access memory using interleaving, wherein memory access is distributed across three channels. If two memory DIMMs are used per channel, there is a reduction of maximum memory bandwidth for this configuration with UDIMM by some 5% in comparison to RDIMM.

Usually, the motherboard must match the memory type; as a result, registered memory will not work in a motherboard not designed for it, and vice versa. Some PC motherboards accept or require registered memory, but registered and unregistered memory modules cannot be mixed. There is much confusion between registered and ECC memory; it is widely thought that ECC memory (which may or may not be registered) will not work at all in a motherboard without ECC support, not even without providing the ECC functionality, although the compatibility issues actually arise when trying to use registered memory (which often supports ECC and is described as ECC RAM) in a PC motherboard that does not support it.

Clocked Unbuffered DIMM (CUDIMM) and Clocked Small Outline DIMM (CSODIMM) modules include a buffer on the clock bus, but are otherwise unbuffered. This enhances clock integrity at higher speeds at relatively little added cost. They were introduced for DDR5 in 2024. The clock buffer is also called a client clock driver (CKD). Systems that do not support CUDIMM/CSODIMM can still use them after a BIOS update that allows for using the modules in pass-through mode.