<p>This article discusses the correctness of one linear inverse problem for the three-dimensional second-kind, second-order mixed-type equation with semi-periodic boundary condition in an unbounded parallelepiped. The existence and uniqueness theorems for a generalized solution to a linear inverse problem with a semi-periodic boundary condition are proved in a certain class of integrable functions. The “<InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(\varvec{\varepsilon }\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="bold-italic">ε</mi> </mrow> </math></EquationSource> </InlineEquation>-regularization,” a priori estimates, approximation sequences, and Fourier transform methods are applied.</p>