In the production of natural gas, (mono)ethylene glycol (MEG) is commonly added to the well stream to prevent the formation of clathrate natural gas hydrates. A reduction in the amount of MEG required for hydrate prevention in industrial subsea flowlines would decrease the costs associated with natural gas production. Methyldiethanolamine (MDEA) is sometimes used for corrosion control in wet gas flowlines by increasing the solution pH and will typically be injected with the MEG. In systems where both hydrate and corrosion control are required, hydrate inhibition via MDEA could represent an opportunity to reduce the required MEG injection rate. However, no experimental data are available to quantify the degree to which MDEA may act as a hydrate inhibitor either in isolation or in the presence of MEG. In this work, we report 20 measurements of the hydrate phase boundary in the presence of MDEA (3-7 vol %) and MEG (0 or 20 vol %) performed at high pressure (6-9 MPa) in a sapphire autoclave cell with both ultrahigh purity methane and a natural gas mixture. The results illustrate that MDEA acts as a hydrate inhibitor and, when combined with MEG, provides additional inhibition. For the systems studied, the effectiveness of MDEA as a hydrate inhibitor is approximately half that of MEG. When 20 vol % of MEG was added to the aqueous phase, the MDEA became less effective as a hydrate inhibitor. However, 7 vol % of MDEA still caused an average temperature shift in the hydrate phase boundary of 0.3 K, which is equivalent to the effect that would be achieved by increasing the amount of MEG in the system by 3% (i.e., from 20 to 20.6 vol % of MEG).