Structure and spectral properties of the hydrogen molecule confined in a spherically symmetric harmonic oscillator potential were studied using the configuration interaction method. Increased strength of the confining potential exerts significant influence on the geometry of the molecule as well as on the vibronic transitions between the electronic ground and lowest excited singlet states. With increasing confinement strength the bond length shortens, the absorption and emission vibronic bands become blue-shifted. and the intensities of the lines change in consequence of the change in the number of vibrational levels accessible in the transition. The spectral changes are noticeable even for weak confinement potentials.